... because I've been running Vista on > 9 machines (work & home) and have been helping various family members get Vista on their machines too.
WITHOUT exception, Vista has been 95% problem free on EVERY one of those machines THAT I INSTALLED. My father in law's Sony laptop was installed with a HORRIBLE Vista image buried under mountains of crapware. Once that was remedied (clean install sans crapware) it was a whole new machine.
The only issues we really saw was when my Mother In Law's favorite writing app wouldn't run without admin rights - the developer was writing app settings to the HKLM portion of the registry. All offending apps were quickly fixed/patched shortly after RTM and even x64 drivers quickly appeared. My biggest gripe was that it took Line6 SO LONG to get their x64 drivers built and released.
So whilst Vista was absolutely fine, Win7 has been nothing but stellar. Light, fast, responsive, it gave a new lease on life to several previously mothballed machines and is a joy to use.
Can't wait for RTM and one last upgrade/install-fest (for a while at least)