It's a Beta you have compatability issues thats a no brainer. Some games will not work some software will not work. Internet is using IE7 which looks like firefox still has it's bugs but I like it (yes I said I like IE for the time being). Microsoft's statments claim that
XP boots 10% faster than Win2K and runs apps 15% faster than Win2K
Vista boots 15% faster than XP and rns apps 10% faster than XP
I'm sure thats what they are aiming for as for what percent it is faster currently I cannot say.