The unit testing suite that comes with Visual Studio Team System is adequate, but I would say only use it if you are using a full-blown TFS implementation and are going to do continuous integration builds using TFS. The only benefit I can see for MS's unit testing suite is that it integrates nicely with TFS builds and can provide some decent reporting.
That said, if you are using only Visual Studio and will be doing CI builds using CruiseControl.NET or TeamCity, I'd recommend using MbUnit or NUnit.Net. In general I think most of the open source unit testing frameworks have comparable features so it's probably not a big deal which of them your choose.