I went with the Küat NV 2.0 with the Add-On in June. Since I just went through this process, I'll share my opinions of the various brands I looked at as well as review the NV. I shopped at REI because they'll take anything back, and my search was largely limited to what they displayed on the floor, so I could play with them.
In my mind there are three top tier racks, the Küat NV 2.0, the Thule T2 Pro XT, and Yakima's DrTray. The DrTray was my runner up, it's got a great handle system and a nice lock attachment point, but it only holds three bikes (I seriously considered tying a bike to the roof and going with this rack). Thule has a plastic fetish that I just can't get behind, maybe their racks are fine, but if the plastic breaks in five years will there still be replacement parts?
The next tier down from that has the same brands again, the Küat Sherpa, Yakima HoldUp, and the Thule T2 Classic. All of these are scaled down versions of their top tier counterparts, though on the flip side, they weigh less too. I liked the Sherpa's integrated lock system, and their flip-up wheel holders made for a clean look. I liked that the Yakima could adjust the relative position of the bikes so that you can keep them from scratching each other (I worried needlessly that the NV would have this problem). And once again Thule is plastic city.
In the end, I chose the NV 2.0 because it held 4 bikes, had a decent lock system, was made of solid steel, minimal plastic, and it came with an integrated bike repair stand. Having used it for about three months, I've got praise and complaints. I really like the bike stand. I thought that this was kind of gimmicky, but actually I use it a lot at the trail head, and at home too because I don't have a dedicated stand. The wheel clamps work well, and Küat will send you a free kids bike adapter if you need to adapt a 20" wheel.
Annoyingly the NV has some thin laminated plastic to protect the paint where the wheels sit on the metal support bars. This has bubbled up and torn in a couple places. Thule's plastic would not have this issue, so a point to them. Also, while the NV's release lever is a nice solid lever which is easy to use for raising and lowering the rack when empty, when fully loaded with bikes, it's difficult to reach in order to tilt the rack back for car hatch access. I find that I have to lift the rack with one hand (or shoulder) to relieve tension on the lever before I can move it using my other hand, which is an exercise for plastic man. When it does lower, the rack rests on the ground instead of the stops. In 2 bike mode this lever is easily accessible and the rack tilts back and rests on the stops.
The Add-On attaches easily with two long bolts which run along the inside of the center support bar. It would be easy enough to pop the extension off, but honestly I wouldn't bother unless I knew I'd be going many trips without needing it. You'd probably want to lube the bolts so they don't seize. One thing to watch out for is that in the 4 bike configuration your exit angle is very limited. I scraped the crap out of my rack on day one, and have had several fun-filled scraping sessions since. I had zero problems in 2 bike mode. The rack easily clears curbs though, so it makes it easy to back into parking spots with a sidewalk or small grass divider and not worry about having your rack blocking traffic. I got my Add-On separately from my base, and the keys don't match. I hear you can order a kit to fix this, but I have no problem with the mismatch as the keys are numbered and I know which goes where.
