I've used nUnit for years, but when we moved to VS2008 and TFS 2008 (using TeamBuild) we decided to try MSTest. No huge complaints there... we really like how well it integrates with the IDE as well as the CI build server.
One new thing we're trying which looks to have awesome potential is another add-on from Microsoft Research called Pex (requires VS2008). As they put it: "Pex generates unit tests from hand-written parameterized unit tests through automated exploratory testing based on dynamic symbolic execution." The way I put it is: this thing does static analysis on your unit test and target code and codegen's unit tests to achieve super high code coverage (which is often impractical if you're doing it by hand).
