There's always plenty on ebay, but they are not perfect. The main problem is the heat buildup, these things have PLASTIC heatsinks and a thermal pad that makes contact with the metal stand to draw heat also.
After 30 minutes of intensive use (iplayer) it will hit 71 degrees and throttle down to 800mhz, which makes it almost unusable. TVCatchup doesn't run smooth enough, even in low quality. Still get occasional crashes/hangs/google chrome using 100% CPU/ubuntu maximus bugs and have to ssh in and kill stuff.
The touchscreen isn't very sensitive near the edge of the screen, and you just can't customise ubuntu enough to make it really easy to use on such a small screen.
In saying that, it's still very good, this thing has gigabit ethernet, 802.11n wifi, 512RAM, stereo speakers.
For £50-£70 they are very good, the only extra you need to buy is a 4GB flash stick and use synergy/vnc to remotely control it. Some people have bought usb hubs to add bluetooth/keyboards/hard drives etc.
I think the 2 main reasons for the abundance on ebay is 1. the price (£50) and 2. people buy them and realise they have no use for them.
How are you doing iplayer through it?
Using an ubuntu image
http://www.stephenford.org/joggler/
This is regular ubuntu:
http://www.fuzzylogic.co.uk/
Development has been idle on both of these images for months now but they work well.
Someone has recently managed to get an ubuntu 10.04 image working, main issue being Intel and lack of GMA500 drivers, probably why the other guys gave up.
http://www.joggler.info/forum/viewtopic.php?f=36&t=1304