Yep, they're pretty good. I'll probably play with it a little more later to figure out what the rough weights on correlation are.
The Twitter API rate-limiting makes things harder, but as a whitelisted app we haven't (yet) run into any situations where we've hit our quota or get throttled in any way.
It sounds like Alex is working on a new method to retrieve a users followers/friends in bulk which should help out a lot: http://groups.google.com/group/twitter-development-talk/msg/...