Both of these products rock, though I suspect using the latter is a task most developers won't find interesting until they really get into the nasty stuff of unit testing legacy code or code that is virtually un-testable.
In any case, it seems like a nice deal to use if you are getting into unit testing. Of course, you can still use whatever unit testing framework your hear desires - NUnit, MbUnit or whatever. These two tools are productivity enhancers more than anything else, and save you lots of manual work:
TD.NET saves you the trouble of running your tests from outside the IDE, debugging into them, testing them under .NET 1.1 etc..
TypeMock saves you from having to Refactor your code into testability - which sometimes is a very practical approach, even though you should be careful just how much you abuse this tool's power.