Today is the official day where Visual Studio Team Foundation Server Release Candidate is out and this was the key of a discussion: all that are waiting for Visual Studio Team System's family but... do you really always need it?
If you're a large company that develops software in team, needs advanced source code control, code profiliing and analysis, integrated unit testing, project management and test case management features, maybe Visual Studio Team System is the right choice for you.
But if you're a small company or a single developer, are you sure that you've to place your money on VSTS?
This is only my personal opinion but the features that are on Team System are not a needs for everyone and there are other free tools that can help you on unit testing to consider.
Visual Studio Team System is like a great Ferrari: all loves to try a Ferrary during the life but maybe at the end the choice that best feets their needs is a Mercedes like Visual Studio 2005! 
P.S. I've said Mercedes because VS2005 now has lots of features that makes it a "full optional" great car... 