Strengths: Incredible stability, freedom of movement, greatly reduced shoulder and upper body fatigue.
Weaknesses: Lack of a externally accessible organizer compartment for tools, bladder not included, bladder access is in the main compartment (an inconvenience, not a weakness)
I got a BD-2 (small mens version) about six weeks ago, and have now done over 500 miles of all-mountain riding with the pack, including several 40+ milers. I'll repost this review with pics in the forums at http://forums.mtbr.com/showthread.php?t=352127
The Ergon pack is built around a semi-rigid nylon frame and a shoulder “harness” that connects to the pack via a ball joint. There is a waist belt with wide pads over the hips and and rear strap adjustment that allows the pads to be correctly centered over the hips. This is important as most of the weight is carried on the hips when using these packs. The front adjustment also has an elastic component that allows about an inch of extra stretch when needed, but I found the pack a little loose around my 28" waist until the elastic was fully stretched.
The shoulder harness comes in different versions for men and women, as well as different sizes. On the front of the shoulder straps are velcro hold-downs to keep a hydration bladder hose in place, as well as chest strap that can be slid up or down on a type of "rail" on the front of the strap. The chest strap also has an elastic tensioner, and the buckle has a built-in whistle, a nice little safety feature. The shoulder straps are quite comfortable, and more importantly, barely noticeable even with 16 pounds of gear loaded.
The shoulder strap setup is more like a harness since the straps don't connect directly into the pack. They're snug around your shoulder joint, but not restrictive unless you crank them down too tight, which is unnecessary since their not as weight bearing as with other packs. The ball joint that connects the shoulder harness to the nylon frame allows for some incredible upper-body freedom of movement, and is the heart of the system. The ball joint itself has three possible mount points allowing for longer or shorter torsos. It's a simple nylon-cap nut and hex bolt that holds it together.
The pack itself is quite unusual in it's design. There are two external mesh pockets on either side, one external zippered pocket on the back, but it's small and not very deep. There's also an internal zippered pocket that's vertically oriented and a little deeper. There's an stretch pocket next to the main zip on the inside of the pack. The bladder has it's own pocket on the inside of the main compartment of the pack, along with an elastic retention system to keep it in place. The main compartment zipper runs vertically, and runs the full length of the pack.
One nice feature is that the pack will stand up, supported by it's nylon frame, when you set it down on level ground. That also allows quick and easy access to the contents in the top of the pack. I first thought the vertical zipper would allow things to fall out when the pack was standing up, but it seems pretty good.
There is a cinch-down loop on each side of the frame. I haven't found a use for them, as with only one strap, a pump is not really secure, and they're too small to hold armor. I tried strapping a camera case there, but it seemed a little exposed in case of a crash.
The last notable feature is a flap/cover that has elastic two elastic connectors at the top, and snap buckle and cinch strap at the bottom. The flap itself has two pockets but I haven't used them to store anything. They seem a little exposed for electronics, and anything bulky seems awkward in the pocket as there is no gusseting. The flap does hold pads or a windbreaker pretty easily, though I haven't tried strapping on a helmet.
The only thing really lacking from the pack was a separate large externally accessible pocket to allow organization of tools or spares. At first I thought I'd miss this but I ended up putting all my tools and small items in a clear ziplock, which works well. Pull out the clear ziplock and I can see all the contents clearly and access them pretty easily.
There's also a rain cover that is stored in a pocket on the bottom of the pack, and can be pulled out and cover the entire pack. I haven't tried the cover with anything loaded externally (pads, helmet), but it easily covers a fully-loaded pack and would be a nice feature for those unexpected showers. But I doubt it would keep out a heavy downpour, and Ergon has another fully waterproof version, the BD-3.
Setting up the pack took some time. Like most people, I've been used to throwing on the pack cinching down the shoulder and waist straps and riding away... not much to adjust. This one took a little more figuring out. First, adjusting the rear waist band so the pads are centered over each hip. Next, figuring out which of the three mount points to use for the Flex Joint. Since I'm only 5' 2" tall, that was an easy guess... the shortest one. I tried the medium mount point just to see, but the straps ended up floating an inch or two above my shoulders. Then adjust the shoulder straps for a snug fit.
I've had to tighten the Flex joint bolt on the pack once on the trail as it had come loose... but even if it does there is no chance of losing the bolt on the inside of the pack as it's covered by a velcro flap. It was on the first ride, so it was probably user error after I'd changed the pack... It hasn't loosened since.
Riding with the pack is where it comes into its own. I'm a pack-rat and carry first aid, food and clothing suitable for an epic even on local trails and short rides. Add camera, tripod, tools, etc, the weight adds up. I've noticed that my typical 16 - 18 pound pack doesn't feel quite so heavy, even though the empty pack is slightly heavier than any others I've used. My shoulders are less tired after my typical 5 hour rides using the Ergon pack.
The pack took about two rides to get used to... my lower back wasn't accustomed to the weight being carried on the hips, but by the 3rd ride, I didn't notice any difference, other than how much less fatigued my shoulders were.
With a correctly adjusted pack, there is no interference with pedaling at all. I remember riding with fanny packs 20+ years ago, and they'd ride down and get in the way unless cinched uncomfortably tight. Not so with this. The hip pads are pretty comfortable.
On my first ride, climbing steep terrain, I did notice the different weight distribution. But by the 3rd ride there were no issues and I was completely comfortable with it. Because of the Flex-joint, the center of gravity of the pack is slightly lower and further away from your back than on other packs. The flex joint also has another advantage... the pack is kept about an inch off your back. Less of your body is in contact with the pack, and there is a little more air circulation than other packs. I wouldn't call the difference really great, but on hotter rides my “wet patch” is around the hips and shoulder pads, instead of my whole back. With the pack heavily stuffed with winter gear, the pack does bulge forward and contact the back, but it's still much less overall contact area than most normal packs.
Get to a technical downhill, and this is where the benefits of the flex joint really become apparent. The upper body freedom of movement is very noticeable. You can twist your body through a tight switchback without any restrictive pressure from the shoulder straps. Leaning, bending forward, and turning have no effect on the weight distribution or tension of the shoulder straps. It feels as if the pack just stays put and your upper body moves independent of it.
On most packs, when you lean to the left or right, or twist your body as if turning the handlebars 60 - 80 degrees for a tight switchback, one strap (on the higher shoulder) ends up taking most of the weight of the pack, and the pack tends to slip sideways on your back. We all get used to that and I never thought anything of it. Not so with these packs...the hips take the weight and the shoulder straps just seem to be there to keep the load stable. The pack doesn't move around, but your upper body is free to move without dragging the pack across your back with each movement.
Hitting jumps and riding rough terrain the pack moves a little, but much less than a regular pack and it's movement is less distracting than other packs I've used. Breathing with this pack seemed a little easier since there is so little tension on the shoulder straps, and at the end of a 35 miler I don't seem to have anywhere near the upper-body fatigue. However, if the shoulder straps are not adjusted correctly, or one has too much tension on the chest strap, there is little to no “give” in the harness which could restrict breathing. But a strap adjustment is all it took to get it right again.
The pack has been through a few crashes, and has held up well, even landing on it on my back once. The nylon frame has gotten pretty scuffed up, but that's a cosmetic thing only. I've not lost anything out the pack. The chest strap has slid up and off it's rail once, but was easy to put back on.
The pack does have a few shortcomings, though these are fairly trivial. There are two outside mesh-elastic pockets, but they're too small to hold armor. They're great for quick access to food/bars or a windbreaker. The rear flap/helmet carrier took some getting used to, and it's hard to put 100% confidence in it to hold my pads, though to date nothing has slipped out. There internal pockets, are a little small and hard to access with a loaded pack. No real organization or separation of tools was possible (at least for the amount I carry) without using something to keep them separate (in my case, a ziplock bag).
Because of the rigid frame, the pack is a little different to get on and off. It takes a few seconds more to throw on the pack and get it set, but those few seconds are well worth it. For my size, the shoulder straps are hard to get my arms through when wearing armor, as it fits more like a shoulder harness than typical straps, so the arm “holes” are much smaller. Again, a minor inconvenience that's easily offset by the comfort and mobility when riding.
It would be nice to utilize the hip pads/waist strap for a camera or cell phone pocket, or other gadge pocket. As it is I can mount a camera using the belt strap, but it's a bit of a hassle and could be done much more elegantly.
Bottom line is that I'd have trouble going back to a regular pack. Fit is important and as noted it took a couple of rides and a few tweaks to really get it set right. I haven't seen any other pack that comes with so much adjustability, as well as the gender-specific builds and sizing. It carries more weight with less fatigue and virtually no upper-body restriction.
It seems they've put most of their design efforts on the ergonomics, and still have a few tweaks left to make on the functionality of the pack. The few shortcomings it does have are far outweighed by the comfort and freedom of actually riding with one. Carrying weight on the hips is a different sensation to what most people are used to these days, but for those who remember riding (or still ride) with a fanny pack, this is much more comfortable and stable than a fanny pack. With all that in mind, this pack probably isn't for everyone, but I'm sure that many, like me, will find it an incredibly functional and comfortable pack.