Vanilla-looking boards are great if you want quick info and not planning on coming back. Full-skinned boards, especially ones that match the main site's layout, make the experience more personal and actually encourages more people to come back.
But note that no matter how good a site looks, it's still
content that determines if people will visit again or not. Even the most vanilla-looking board can have a very active community if they offer a lot to members (prime example for me is my friend's site,
http://www.thehelper.net/forums/). Forums that are all aesthetics and not much substance can have the opposite effect.
However, compared side-by-side, I usually come back to forums that reflect the attention to detail that the forum administrator has done, both in content and in aesthetics. I always appreciate hard work, and more so when they think more about the visitor's surfing experience rather than their own desire to cram everything in
- T