MbUnit is a Test Driven Developer's best friend! With powerful features like db rollbacks, row-based testing, expected exceptions this unit testing framework is the leader of the pack.
The development team does a great job of addressing issues and adding new (useful) features. Give MbUnit a whirl.
First, I was using NUnit and I was pretty happy with it. Than I tried MSTest. I just tried it and went back using NUnit pretty fast. I was happy using NUnit until I read Scott Hanselman's article "MbUnit - Unit Testing on Crack" (http://www.hanselman.com/blog/MbUnitUnitTestingOnCrack.aspx). I don't think I'll be using NUnit again. MbUnit is by far the best testing framework for .Net. MbUnit doesn't stop to impress me; I'm constantly discovering new features.