If you are on Vista32 then its not worth it.
If you are on XP then the differences will be fairly sizeable in many ways and not noticealbe in others... Depends on luck a lot of the time as to whether or not it helps your particular PC out.
Im lucky enough to own a ridiculous number of different PCs of various flavours and on some, Vista is a hell to work with, and others its a dream. Annoyingly, my Main PC just hates it, and I want to move to it so much, but I just cannot.
I have compared the 4 top Windows O/Ses ( Vista and XP in both 32 and 64 bit guises ) and quite simply, XP64 is the slickest and fastest of them all, and I did this on 3 different setups ( Low+Hi end C2D & Dual Opteron ) but in all cases no matter what PC I used, I ound that Vista64 was much better than XP32 - even on slightly older systems in fact.
Software compatibility?
99% of software will run 100% fine
Sure, you will have a few niggles, but then Vista32 has has its fair share of niggles as has XP64, this is normal and these niggles are gettign fewer and farther between each other... My own opinion, is that absolutely every game I have ever tried, runs better on a 64Bit O/S than its 32Bit counterpart, although I know of others who said that the games dont work, I have never found one that has failed me.
Oh, thats a lie... Bet On Soldier wont run on a Vista 64 MAchine because the O/S is apparently too old?