bbPress is great for what it does. It's lightweight, and just works with WP. Plus there are numerous plugins to expand bbPress' functionality.
The thing is, it's gotta be themes. It works well with default WP themes (2011, 2012) but it does look pretty basic - and results vary with different themes. To get it looking just right outside of the default themes, you'd have to find a theme dev who supports it, or do some custom theming.
For really in depth forums with lots of features, you probably want to look elsewhere. simple:press was good back when I used it - but it had so many features I didn't need that I eventually switched to bbPress.
It's difficult becasue most themes don't account for any forums, and most forums can't account for every theme. So it often ends up looking like the forum is kind of wedged in or whatever.