We've got an MS SQL 2000 database server running internal applications (< 50 concurrent users, tables up to a few 100K rows, some web, some .NET clients). We have no intention of shelling out $$ to upgrade the server, and are trying to decide whether to leave it on SQL 2000 or migrate to MySQL 5.1. I've used MySQL for smaller installations, and have always liked it. The pros of migration from my standpoint are that I like writing queries in MySQL more than MS SQL, and we can afford to run the most up-to-date version of MySQL. The pros of remaining on MS are better integration with our active directory environment and better .NET interoperability. I'd be interested in hearing others' advice on this, and further pros (and cons) about going either way.
No flames please, I like both MS SQL and MySQL and believe they both have their place.