It depends...I was a Beta Tester and as being a Launch Customer for MS got free licences. Here is my summary:
1) Main Family PC (Intel 6300 Dual Core, 1GB RAM, 1TB of Storage) - Running Vista Ultimate & Office 07 & MC: This is a winning combo especially due to the Parental Controls (we have a 10 & 8yo and this system is great...no it really is great!). This is our main PC and I also use it to back all my HTPC media. It is stable, quick and on a newish PC I had no problems with drivers etc....a big improvement on the RC's & Betas (note: I also had Vista on an older PC and it caused nothing but problems). I don't use MSMC primarly as MC's flexible codec selection and support means I can play all of my media.
2) HTPC (Intel PentD, 1GB RAM, 1TB of Storage) - Running XPSP2, MC, Codecs etc: A dedicated HTPC, I really don't care about the OS on this box as long as it is quick, stable and runs all the bits needed! On this box I don't see any advantage to changing to Vista.
Nathan