Couchbase is now our first choice for NoSQL for many teams/groups/departments in Western Union. It serves a purpose for a quick and easy transition from old expensive RMDS databases to the new generation of cloud NoSQL storage.
Couchbase is easy to set up in the cloud or on the on-premise servers. Within minutes you can get yourself a database that will cheap, conveniently accessed, and highly available. More than that - it'll retain the way to query it like a traditional RDMS.
Couchbase is easy to maintain because high availability is one of its main features. You can build your Couchbase cluster on, say, 10 virtual boxes in the cloud, and have all the work on the database side done without any interruption to your applications.
Couchbase has great customer support. They are always ready to help when you face a problem. Tickets opened easily and get a quick response on support.couchbase.com. Documentation is awesome.
N1QL query language (very close to a standard SQL) is great when your application cannot rely entirely on key-value gets. This makes it easy to convert the tables of legacy RDMS and applications accessing it to the new generation of apps-database combo.
General recommendation from Couchbase would be to have RAM on servers big enough to fit 20-25% of a dataset. This is promised to be driven down to just single percents with release 6.5 in summer of 2019. This can drive the cost of underlying servers dramatically for huge TB databases.
Indexes are tied to specific servers. It'll be nice at some point to have them spread across all index nodes and be able to rebalance indexes the same way as data itself.
Very scalable. Needs more capacity - spin out new EC2s and add them to the cluster. All within minutes, all on a live database. Similarly - you realized you're spending too much on database servers while having more than enough RAM - downsize your EC2s or simply take some out one by one without interrupting the cluster.
Couchbase will be best when quick access is important and the size of the database is not huge. For huge databases (say, greater than 5TB) the cost of underlying servers can be considerable. Couchbase is very good when high-availability is important. Also, Couchbase is "the quickest of them all" (as I say for some independent comparisons).