I've worked heavily with MySQL, SQL Server, and DB2.  I've never been able to fully embrace MySQL.  The lack of features you come across prior to some of the newer versions just left a bad taste in my mouth.  It's fine if you need something free, but I don't think I'd want to use it for a database at a for-profit company.  I have mixed feelings when it comes to DB2 and SQL server, and I think a lot of the decision comes down to what the database is being used for.  If you want freatures, and easy access, then SQL Server is the way to go.  if you want bulletproof stability, and need absolute 100% uptime, DB2.  It's not that I think SQL Server isn't stable, it's just that DB2 on an AS400 is a database server built on an OS, and hardware designed to run a database, while SQL server is a database server built on an OS, and hardware designed to do a lot more.  The additional complexity, of the underlying systems add a level of instability.  As far as unplanned outages go, our externally facing SQL Servers rarely have a problem, but our internal SQL servers that get nailed with ad-hoc queries, reporting services work, etc have their fair share of problems from queries going wild, and killing performance to out and out crashes.  We simply don't see those same issues on DB2.  our unplanned outages from our AS400 over 6 years consist of 3 times the power stayed out for an extended period, and once that we lost 3 drives on 2 arrays in a 24 hour period.  As for slow down, even with badly written ad-hoc queries, things will rarely show a marked hit on performance.  I'm not positive as to what is really different, but, there is definitely a difference.

I'm not saying SQL server is bad, our slow downs from SQL server are only ever a show-stopper on what are already massive processing jobs that run slow, and our down time is usually measured in minutes, and happen maybe once every couple months.  not at all bad, and you can probably blame windows for 99% of the outages, but, it's still not as good as DB2.  However, very few applications really require DB2 on the AS400's level of stability and up time. 

Not to mention, us DB2 Admins really get no respect... When the system just works, works well, and never goes down, execs and users tend to forget exactly how mission critical it's function is, if not completely forget the system is even there, or more important, that the admin actually does do work on the system to keep everything running so smooth

And don't get me wrong... I far prefer programming, and working with SQL Server...  It really is a much better platform to develop on then DB2.  However... managing DB2 can be a much more pleasant experience...