As others have pointed out, PostgreSQL is very good in terms of features, speed and reliability. However, after having developed an enterprise .Net application in it, I have to say that it does not mesh with Visual Studio as easily as SQL Server. Aside from that, SQL Server vs PostgreSQL is a little like the Windows GUI vs the Linux shell: one is easy to use, but the other is much faster and more powerful, ONCE you get used to it.
Apparently you can't use LINQ via the ODBC driver, and if you're developing a system using sometimes connected mobile devices, you lose the option of automatic synchronization that you would otherwise have with SQL Server and SQL Server Compact.
