Here’s an interesting article reporting the results of testing SQL Server Express, MySQL, DB2 Express, and ORACLE 10g Standard Edition. MySQL generally comes out on top but the report is worth reading for what else it says.
Read it at Road test: Four databases tested.
Keep in mind that they were testing the “lite/express” versions of the commercial vendors but the “regular” version of MySQL so don’t read too much into the results. While enterprise databases are not in much danger, if you use an embedded database engine in your product it’s worth rethinking in favor of open source engines. One major drawback of the article was that it didn’t test PostgreSQL.