I've never had a bad experience with it, it needs a lot less maintenance than Oracle does (or did, at least), it performs pretty well (we're an insurance company with large datasets), is integrated in a .NET stack (while you might not want to go there, especially as a startup, it is very nice for corporate work), it came with Reporting Services which we used to replace Crystal Reports, it has lots of (admittedly non-standard) extremely useful SQL functions, data types, etc. And I love SQL Server Management Studio and the other Microsoft tools.
MySQL felt like a toy database in comparison (especially the management aspect), and the non-relational stuff is out of the question for now. We might not have done due diligence by not looking at other alternatives (notably Postgre I guess) but they seem like a poor fit given our developer's strong Microsoft-centric backgrounds. Most of us also have at least one Microsoft training course in their SQL server, and they have very strong support in my country (Uruguay) against nonexistant for most other platforms.