The design part is a matter of skinning / theming; there are several themes out there, and it's growing (fortunately). The integration with WP is not smooth sailing, but then again:
- bbPress is technically still in pre-release
- it works well as long as you use WP 2.5.1
- if you like bleeding edge, there's bbPrss 0.9 alpha
I've seen things to embed other forum solutions, but there's the issue of integrating the user database, and getting themes / skins synchronized is a pain in the, uh, back.
I honestly think bbPress is your best bet - if only because it can handle traffic well, integrates relatively well (if you're sticking to the stable WP 2.5.1 release), is backed by a core of WP coders, and is much easier to tweak (theme-wise) than most any other solution out there.
Still, the small group of bbPress coders is in need of help; if you can help, too, that will speed up the first full release - which aims at working alongside with whatever the current, stable WP release is at that time of release (no idea which one - don't think it'll be WP 2.7 though).