CouchDB was a very positive experience at first, and its reliability isn’t out of bounds for a database that is, after all, only on version 1.2. But our service is very sensitive to reliability issues, and ultimately we decided that this transition was the most efficient path forward for us.
Once we decided on MySQL (specifically, we’re now using Percona, with its InnoDB-based XtraDB storage engine), we rearchitected our DB abstraction layer and one by one migrated all our databases, large and small. Our uptime was dramatically improved over the past couple months as we worked through the migration, and performance was slightly improved in the bargain.