Entraspan has been using MySQL since its introduction.
MySQL is great for smaller databases, and especially databases with a high transaction rate where the cost of having a dedicated licensed database solution would be too costly.
One of the features that I wish was available out of the box in other databases was the SQL replace statement.
Downsides of MySQL has been poor subquery performance, and the query optimizer isn't nearly as good as SQL Server and Oracle. Also it is not that hard to hammer the system with an ad hoc query that requires full table-scans.
It took forever for MySQL to add foreign keys, and it still is a problem that to alter a foreign key on a table requires the entire table to be completely rewritten, which pretty much takes the database offline.
Recently a large database load of about 40 tables and well over 1 M records was done side by side with SQL Server and MySQL on identical hardware. MySQL killed it on the inserts, but at the end when the indexes/foreign keys got added, each alter kept rewriting the tables and made SQL Server the fastest load.
Apparently MariaDB has greatly improved the subquery performance, and I would expect to move more databases away from SQL Server in the future.