SQL Server certainly has some quirks, but what mature database platform doesn't have them? Having worked in Oracle for years and then SQL Server for years, I have to say I prefer SQL Server. Then again, comparing them is like comparing apples and oranges. Sure, they're both members of the same family of software, but they're very different products. They just work differently.

With the advent of the many different relational database platforms out there, I think SQL Server and Oracle are still the major players. The reason is that they're very good at what they do. The inconsistencies that Phil pointed out in key areas make me hesitate. While flexibility is good, so is consistency and reliability. Any product that tries to be all things to all people isn't going to be the best at anything.

The family of document databases is growing, as are the free, open source databases. I honestly don't know enough about them to say which one's the best at what they do. I don't even know enough about them to discuss the quirks, if they have any yet. I'm guessing that as they grow up, they'll have quirks, just like any mature platform.