Didn't go with the Xebec because I happened to be shopping with BB that day and they had two versions, so I got one of each.
NFT
This was a single piece that went on easy enough, but not super securely. Definitely wouldn't work if it was sitting in your lap. You would have to have your laptop on a table, which kind of defeats the purpose of a laptop. Very heavy and big power requirements. Mine had a piece broken right out of the box, so that sucked. It was a knob to tighten something, but I'm not really sure what. It works, but there are big drawbacks. I mean, why not just get a desktop and three monitors?
Mobile Pixels
This came as two separate pieces. It's really just a second monitor, but you can put one on top of the other to get to three. (Theoretically, I guess even more.) Lighter than the NFT, but goes onto the back of your laptop monitor with small adhesives. Thus, it feels a bit flimsy, but so far the two laptops I have them on have no failed. Did I mention I just used them separately instead of as a tri-monitor setup? Now I have two laptops with dual screens (I'm not the primary user of either.) The users are pretty happy with them. The monitor itself slides in and out of the housing that is secured to the back of your laptop, so that's cool and makes it more portable.
Right now I'd recommend the Mobile Pixels one, and you could probably just get a solo unit if you were just adding a second screen. However, looking at the Xebec again, I think I'm going to order it. Seems to have the best of both worlds now that I know what I'm looking at. If I do, I'll review it when I get it.