I had a Top Fathom skimmer a few years back -- I wasn't too happy with it either. It was really dependent on the amount of water in the sump -- it had to be just right to get good skimming. I know that the newer models had some sort of auto-leveling device that was supposed to help with this problem, so if you choose to get one try to get one with this device (although I can't guarantee it will be any better).
Basically Top Fathom skimmers use "older" technology. I'm not saying it's bad -- it's just nowadays you can get skimmers that are more efficient, easier to use (set it and forget it instead of constantly re-adjusting your skimmer), smaller, and less finicky about placement.
I think the reason you may find these for cheap are because people are switching to the newer technology that I mentioned. If you're willing to work a little bit extra to get the thing working and keep it working the way it should, then it's a pretty good deal. However, I'd personally opt for one that is easier to use.
D-Nak