I have personally used BBPress and integrated it with WordPress. There were some teething issues, but once set up it runs well and has some nice cross over features with WordPress, such as posts on the front page and users only having to log in once.
It is hard to skin though, and isn’t supported that much. A better option is VBulletin, and that connects with WordPress to some degree too, as you can create new forum threads from new blog posts on the fly. That’s a better forum.
As for SEO benefits, massive. So long as you have an active community and keep the spammers at bay, you can get great ranking benefits from a forum.