To each his own - I built a full HTPC just over a year ago just before Vista was released. I therefore qualified for a free upgrade to Vista. I have not upgraded this HTPC since it took a lot of playing around and configuring to get everything working properly. Plus the UI in Vista Media Center is slightly different. I don't want to tick off "She Who Must Be Obeyed" by having the system not work for a while because I am playing with video drivers, etc. and then have her get frustrated by the new interface. I also have bought 3 (soon to be 5) extenders that allow me to play this content on other TVs around the house and these extenders don't work with Vista, only XP MCE.
But I do have Vista on another PC and, for the most part, I like it - it looks gorgeous.
Assuming that you still have a license for Vista you could always try installing it in a dual boot config. It probably isn't worth spending $100 for XP MCE plus it would require an OS reinstall.
The other things you could try are other HTPC software - I once tried something called Meedio and it was good but then it was bought by Yahoo and killed. There are other options out there - MediaPortal, etc. that act as a front end for media content.