Cloud SQL itself is fine...it's just hosted SQL. That said, there is a lot of google-bespoke code for interfacing various parts of their cloud with it. Cloud SQL Proxy has a lot of gotchas and hard to debug. For example, the per-instance connection limit between app engine and cloudsql is 5, and it's impossible to know how you're bumping against that. Tuning is a frustrating process, and google support hasn't been too helpful.