We don't use Drizzle directly, but have been using lib-drizzle to give us c library access to MySQL. In almost a year, we haven't seen any negatives.
A good point, quite a few people are using the libdrizzle client library bundled with Drizzle. It is a high-performance BSD licensed library that can speak the MySQL protocol.
beyond blogs (my wordpress blog included), forums and mail servers I don't believe it is used in a production environment yet. But Rackspace intends to use it in production environments.
We believe it is stable enough (hence the GA) our testing and regression suites are very comprehensive.