I have 2 of these, one for a server and one for mirror backup of the server. There are quiet, very small, easy to set up, very reliable, and I like being able to hack it by installing custom firmware. It is very stable, and I haven't had to do any administration on them since I set them up 9 monhts ago. However, I find them fairly slow over the network. There is a noticable lag getting data. It's not terrible, but you know the files aren't local. For comparison, just last night I created a beefy Debian server (Athlon64, 1GB ram) that serves the same files from IDE-connected disks, and files from that server come up in an instant, and the response time feels close to that of a local disk. Obviously, I prefer the faster server, but it is an order of magnitude more expensive and at least an order of magnitude more complex to set up.
As long as you don't expect high performance, you'll be happy. I'll probably keep one of mine around as an iTunes server or for some other fun hacks. After all, $59 isn't a bad price for a linux server the size of a paperback book that has 2 usb ports and an Ethernet port. It seems like there should be all sort of interesting things you could do.