I've used phpBB for a couple years or so. Personally, i think its great; pretty easy to customize the look & feel and plenty of mods.
...just make sure you keep it patched and use all your registration confirmations (email activation & graphical verifier) or you'll end up getting BOTS from Russia wanting to sell you Xanax, et al.
By the way--you can see an example of 1 of my phpBB boards at:
http://www.sceneorlando.com/