I switched my commenting system from disqus to intensedebate.
They're pretty much the same. Both of them feature a wordpress plugin which keeps the comments in the WP database and in the commenting system the same, both of them have threaded replies, ratings for posts.
The main difference as far as I've seen:
- intensedebate has REAL openid support (a BIG plus imho!)
- intensedebate doesn't create (imho: stupid) "communities"; that basically are just the comments themselves
- intensedebate has a real openid signup (no "extra"; password, embedded via API key)
- easy RSS tracking of comment threads
- Gravatar support
Disqus might have some of those too, but I haven't seen them the time I used it :)