I would say, if you're happy with what you've got, stick with XP
Not that's anything wrong with Vista, I've upgraded myself. The problem is that "while I'm upgrading... I might as well upgrade a few components while I'm at it" which quickly turns into a new PC :-D
There are benefits to upgrading - The TV Pack is one of them, and the hetrogeneous multi tuner support it brings (DVB-T and DVB-S coexisting).
Support is another - most add ons are being developed for VMC, not MCE. Most forum questions are asked about VMC.
I still find the MCE UI more intuitive, easier to use and faster. VMC is prettier, but I still find it clunkier than MCE. There's a picture of someone's MCE setup in the DIY section (I think), everytime I see it, I think "Wow, that looks..." Can't think of a word now, but something that combines simple, efficient, elegant, clear (plain) all into one...
These flashy backgrounds and clever transitions - what do they actually bring to the usability?
Don't let me put you off, though. My point was, if what you've got does what you want, don't wonder that you're missing something by not being on Vista. If you want to have a fiddle/play, go for it.
only reasons i can see are the dvb support and isnt the audio supposed to be better as it does away with the mixer?
but if i just decide to swap my dvt card for a freesat or black gold 6 in 1 one i suppose i could stay with mce