I have a beta account but I have not yet used it very much because I have CouchDB running on two of my own servers, and usually on my laptop.

Except for not really being able to do ad-hoc queries (you write map/reduce view functions which create indices) I like CouchDB. That said, I use MongoDB a little more frequently because it is easier for me to use.

What I really like about CouchDB is the way it uses REST and JSON. I use Ruby clients, and sometimes I prefer to just use simplehttp and json gems, and do everything low level because it feels natural.