CouchDB is fast. It will meet your needs perfectly. It's good for a forum as each post and all related comments/posts in a thread will be self contained. CouchDB maps are, as far as I've seen, faster than MySQL Joins when MySQL has a large dataset.
I would say go for it.
Edit:
If you want an example of how CouchDB can be used in a decent way, check out skinnyboard. It's an agile planning tool and contains tasks on a story, and stories on a board, all with permissions in one CouchDB document. The code is a little messy in some places, but it's a good example of data encapsulation using CouchDB.