SQL Server is a great product, but current licensing costs have pushed it into the high-end price bracket, opening up the same sort of price/feature hole in SQL Server that Microsoft used in the past to attack the other major players.
If you have a large number of applications that require SQL Server, then you are commited to the platform, like it or not. And of all the software Microsoft makes, SQL Server is one of the better engineered product lines.
But if you are not a 100% Microsoft shop and a good chunk of your applications are web based and you are not "platform locked", then there's a host of options that need to be weighed.