I started in DOS 6.22, have always gone to the newest thing as soon as it is released and vista was no different. Through this I have noticed the pattern (and it isnt a suprising one)
1) The new OS always introduces a few new niggles
2) You dont realise it, but you rapdily become used to the slicker interface, then when you go back the old suddenly feels slow and clunky
3) You spend the first hour going "ooo" then after that you forget your using a new system
It was that way when i went from 2000 to XP and then from XP to x64 (though this was even less exciting) I then went to 32bit Vista Business and have recently chopped it in for 64bit Vista Business as I upped to 4GB RAM.
I ran the 32bit Vista from the day it was released in February 2007 without having to format etc etc until two weeks ago when i put on the 64 bit Vista. EVERYTHING worked in the 32 bit one and it became noticably better as the hotfixes piled on through windows update.
64 bit is much the same with two exceptions. 1) Rivatuners low level driver wont install so if i wanted to mod my 8800 to a quadro i now cant and 2) until I had applied all the windows updates the latest Nvidia nforce driver set wouldnt install for my 680i mobo.
I have an X-Fi and it hasnt ever caused me issues in either OS, though you do need to find the Vista compliant software release out there on the net as the OEM supplied CD wont work.
As you have the license, install it, try it and if you hate it go back to XP. You have 30 days to activate it so no need to "waste" the license if it isnt for you. I can guarantee if you DO go back though you'll be horrified by how clunky XP feels in comparison.
One final thing, if you do install Vista then make sure you manually install this:
http://support.microsoft.com/kb/940105
It still doesnt come though windows update (god knows why) and its a vitally important virtual memory fix that causes many many games to have untold problems with Vista.
In short... DO IT.