I'm using CouchDB every day and could not replace it with Mongo if I wanted to. So I guess I don't care how popular it is with others, since it solves my problems today (and it's at least popular enough to be maintained and improved).

That said, CouchBase has a huge messaging problem. It took me hours to figure out what CouchBase and all their editions were about. I'm interested in exploring their products, the write performance improvements should be huge, but their web site definitely is confusing and off-putting.