I've used MySQL extensively for personal projects (all of mine have been at a pretty small scale). I've also used both Oracle and SQL Server for projects at large companies.

There's probably not much of an advantage in the database engine itself, but the SQL Server suite shined in 2 areas: analytics and reporting. The 2 add-on components made large-scale data mining & model-building much simple, and also automated the generation & delivery of complex reports tracking many metrics. I could probably do both with MySQL, but only with much more effort.