I have never seen a WordPress installation that wasn't so overrun by comment spam that it made it worth it to even allow comments in WordPress. We've turned comments off completely and just direct people to the forum to comment on content that originated there.
I must be doing something wrong (despite deploying Akismet and other anti-spam measures) because I do see other WordPress installations with lots of comments. Given my experience, though, I just assume that there is some very active admin who regularly combs through the WordPress comments and prunes all the spam and lets through the legit comments. (That's way too much work, however.) I also see a lot of WordPress installations with comments that are clearly just clever spam. In short, the effort-reward equation with WordPress stinks, IMO, when it comes to housing "discussions."
Someone tell me I'm wrong--or tell me where I'm going wrong with WordPress. I've looked at the WordPress "bridge" products available that let WordPress and Xen share a database. In the end, though, I deem it to be more trouble to implement than it's worth.
I use WordPress as a CMS only; the Register link on our WP page takes users to the Xen registration screen. That's a solution that works for us. Oh, and even if you do get the two systems to share a database, the commenting interfaces of the two products are quite different.