I've rarely had problems on Vista or XP. And once you turn off the overly-intrusive security in Vista it's really not as bad as it's made out to be.
Not switching to Windows 7 right away, tho.
Win7 has been working fine for me on my laptop. It is frustrating where they have moved stuff for no apparent reason and I STILL am pissed that the "up" button is still missing from windows explorer (you can take the boy out of unix but you can't take the unix out of the boy) but in general it seems pretty stable. Granted the release candidate is "ultimate" and I might find myself dissatisfied with using any version other than their overpriced ultimate, but we'll see.
For that matter, the ONLY barrier to me adopting win7 is price, plain and simple. I have 5 machines that would need upgrading and I don't have $1500+ to spend replacing operating systems that still work ok.