Couchbase is well established and reliable, has a track record going back to memcached and couchdb. And they're moving very fast with good features (but the way they do it their paid enterprise edition has the new features while the community edition lags a bit, which is fine because this means the community edition is really damn reliable and high quality.)
It is also really heavy weight. I run RethinkDB on a system 1/3 the size of the minimal Couchbase system and still get excellent performance. The system requirements for Couchbase made it a turn-off for me.
