This is a question that I have asked myself, collegaues and clients now for a while. Next week Vista will be available on MSDN Subscriptions and TechNet and I guess that everyone will be there to download it - me too! I have been running the different betas and release candidates for a while and have waited long for this RTM.
I will not install it on my main work laptop, since I have to use Visual Studio.NET 2003 for a while more, and there are no support for Vista and VS.NET 2003. Vista will instead be installed on a Virtual PC so I can test it there. If I get the time I will do the opposite; XP on a Virtual PC with and Vista as host.