I've used Bearshare, Limewire, and Gnotella.

Bearshare: was kinda easy to use. Limited configurability. Gave up on hosts too quickly. Great for MP3s most likely but sucky for videos or other large files. Also showed that it was using like 25Kbps just for talking with hosts. So I decided to save the company T1. Forgets all the files you were trying to download if you have to close it.

Limewire: slighly more configurable than Bearshare. Seems to use less bandwith for host maintenance/discovery. Very good about never giving up on a host. Unfortunately it gets to the point where it only checks once per hour, so the odds of getting a video downloaded are lower. Forgets all the files you were trying to download if you have to close it.

Gnotella: The most configurable of the 3. Will eventually give up on host after a few hours (better than bearshare, but worse than limewire). Remembers your selected downloads if you have to close. Great for shutting down and playing Eq then starting back up at bedtime.

For me, it's a tie between Gnotella and Limewire. If Gnotella would never give up on hosts like limewire it would win on all points.