I just got mine today. It's the original Lapdock, (not the 100 or 500), and from what I understand, these are the ones that are going for $50. The 100 is around $150-ish, and the 500 is $300 or thereabouts.
Was told the Bionic version would not work with the Atrix 2, even though the port connectors are the same. (The Atrix 4g version won't work with anything but, unless you want to take it apart and flip the connectors around.) However, the Atrix 2 WILL WORK with the Verizon Droid Bionic lapdock. I'm using that setup right now. From what I understand the 100 and the 500 work with a lot more models. However, if you want to spend another $20, you can get adapter cables and Borg it out- should work fine, then.
I upgraded my Atrix 2 to ics, and have Webtop 3.0. It is almost identical to the interface I have on a cheap Polaroid tablet running ics. Read that it's supposed to use Firefox, but it seems to me to be just the regular Android browser. Probably Changed to that in the 3.0 version.
It's a neat toy for $50. Ebay is loaded with all 3 models, but, like I said, it's probably the original one if it can be had for $50 unused or refurb (unless it specifically says it's a 100 or 500 and it'll work with the MAXX, or if you get the adapter cables.). It'll be useless as - is for anything but the Droid Bionic, Atrix 2, or Atrix 4g. Even if you have one of these phones - you'll be sick of it in about a minute if you have, or are planning to get, an Android tablet.