I'd be interested to hear a little bit about choosing between CouchDB, MongodB, and others. Couch and Mongo are the two I'm most interested in, but I haven't had the time to really look at either closely.
Couch is interesting in a way, but I feel like the set of problems that fit it well is far smaller than Mongo's. (and that that set is far smaller than SQL still, but that's another matter)
Mongo is also way way faster, though the API isn't as 'cute' by far. I feel like the great web UI for couch, along with their heavy javascript/json usage is the main reason it gets so much traction.
There is one thing that couch does to better than mongo, and thats single server durability. You can pretty much kill -9 couch and it won't care (that's pretty much how it stops itself).
Now, I understand mongodb's stance on why you should be running multiple servers for your high performance app, but for a lot of apps that don't need HA that's just overkill and a pain to configure (and extra $$$ for more hardware). In fact, I'd go so far as to say thats the case for most apps. If I recall they're working on it at the moment though.