The thing that BBPress did right was posts and such as custom post types. It is so deeply integrated into WordPress that I would argue that it pounds the competition when it comes to finding a forum software to use in a WordPress site.

This is important in so many ways from user management, integration, theming and so on. SimplePress does a good job, but it is still just a shell inside WordPress. BBpress is WordPress.

It absolutely has a long way to go, it is still hard to theme (but has gotten a lot better) and lacks basic forums features (though, for what its worth, various plugins have been covering lately). My biggest issue is the theming. Having built out a BBPress theme (ironically for the Stargazer theme that this site uses), I spent more time taking stuff away before I could start building it up. I'd like to see (and I mentioned it to the devs when I took the survey) is a default BBPress look that is clean and streamlined, one that looks good on its own (a la Vanilla Forums), but lends itself to customization that doesn't require tearing it down first.