I used to use NUnit, but now tend to use MbUnit, for two key features:
1. The RowTest feature allows you to easily run the same test on different sets of parameters, which is important if you really want thorough coverage.
2. The Rollback feature allows you to run tests against your database while rolling back changes after every test, keeping your database in exactly the same state every time. And it's as easy as adding the [Rollback] attribute.
Another nice aspect of MbUnit is that its syntax is nearly identical to NUnit, so if you have a whole test bed already in place under NUnit, you can just switch out the references without the need to change any (very much?) code.
