I have to chime in here. I'm a fan. The single wheel lets me hug the curb, if I'm riding on the street, or it tracks so direct, that I can maneuver singletrack just fine.
I used my (rigid) BOB to carry six sealed lead acid batteries for a 72 volt / 40 amp Crystalyte hub motor for a while. This was about eighty pounds. It was all loaded toward the back of the BOB, strapped down, closest to the little 16" wheel. This rig would go high speeds most of the time around 33 mph on the smooth tarmac streets. (Max speed was about 41, but you really had to crank it to get there) - anyway, I did not have a problem with fishtailing.
Similar to fishtailing, it gets "shaky" at slow speeds and just moving the bike around. Like the weight almost vibrates back and forth, it's hard to explain, but the bike is "jittery" with a load at walkign speeds - once you get moving, it smooths out.
The problem with the "jacknifing" (while walking the bike backwards or loading) was annoying, but once I put the sidestand and the center stand on the trailer - I never had that problem again. Also, you learn to get better at backing up with a trailer and so forth. It's all about both - have racks and baskets, then attache the trailer when you need to haul more.
Yes, there were times when I experienced the fishtailing phenomenon, but it was rare, and I would just slow down. It was not often, and I don't know what caused it. In my opinion, single wheel trailers are HIGH SPEED trailers. I would FLY around with that thing, but I wouldn't feel as confident with more wheels sticking out back there. I could LEAN into curves - these tests were done at electric motor speeds, normal people don't pedal that fast - and I can seriously attest to the BOB's high speed handling, and it's superb.
What I find to be more annoying, that I haven't seen mentioned yet, is that it would bounce. The lighter the load, the more it would bounce - it doesn't really seem to hinder your control of the bike, but it's disconcerting. At one stage in it's life, I had a hub motor in the BOB's wheel, driving the bike with the trailer - I got the idea from something called the "MotoBOB" - when the trailer bounced it would lose traction to propel me, and then when it touched ground again, it would spin out - so the trailer driven thing didn't really work out with the bob - unless you only ride on smoooooth roads. (Keep in mind, I have the rigid version, I bet the IBEX solves most of these issues, but I don't know)
If you drop off a curb, the trailer could get hung up, but I guess that's will any trailer.
I don't know why they never came with a kickstand, but I added both a regular Greenfield sidestand to the BOB, as well as a center stand - on the BOB only. Both of these stands hold the entire rig up very securely, it's actually really cool.
In it's current configuration, my bob became a limousine trailer - about twice the length of a regular bob, with huge motorcycle saddlebags over the trailer's wheel. There's a beefy steel rack welded on there to support it.
The way it attaches at the axle, makes it handle so well. Surly's new trailers attach in a similar way, but they're dualies, and I'm not interested in anything that has more than one wheel. It keeps the leans proper.
The fact that it is detachable, as someone said above, is why I prefer it over a cargo bike. I wouldn't be able to put a big dummy in my bedroom, or on a trunk car bike rack (sideways, on my little Honda - I guess I could, but - no), so if you don't have a garage, a big dummy is going to be really hard to home.
I like racks on bikes, maximizing your cargo capabilities on your main frame first - and only when you need the extra capacity, then you can attach the trailer. This is because when all the racks and baskets and bags are on your two wheels, you can maneuver the entire scenario into elevators, up stairwells, into apartments, etc - much easier than a "big rig" - This is important if you live in limited space and don't want to leave it locked outside.
Oh yeah, and plus one on the dry sack - it's worth at least half the cost. I use it on my motorcycle for long trips too - it's like a huge ortlieb dry duffle or something. I also had it on a 29'er with Schwalbe Big Apples 2.5's, not much clearance, but works just fine - never had a problem - never tried it with knobby 29'ers but the Schwalbes weren't skinny.
I'll try to take a picture of mine later, but until then - here's one that's a million times cooler than mine, which was also partially the inspiration for me to customize my rigid BOB. All credits go to the original photographer/builder - I found this on an image search. He's even got a Chris King Headset for the Headtube of the Trailer's Fork!
Here is the LINK to his blog
