I have to say, that I'm running Windows Vista Ultimate right now, and I think it's pretty nice. I postponed running it since before release because I received Ultimate for free from MS for beta testing. I waited until all of my pieces of hardware had drivers released from their respective manufacturers like my 7800GTX, or my Logitech Keyboard/Mouse, or my X-Fi card.
There are a few annoyances. Such as the overprotective administrative privileges alerts. I understand these are for your protection, but for a general user, these would be very frustrating.
A few of my programs don't work fully because they are not vista capable. But I can still use them enough to where I can overlook the features that don't work. (ISOBUSTER, WINRAR)
Some of the icons for programs look bad because the MS-Vista icons look great because they used a much larger pixel rate to make their icons look better. Overall, I would feel that the common user would not need Ultimate. Vista Premium or even Basic might be a better suited OS for them, unless they wanted the benefits of Media Center and whatnot. But you will pay a premium for those features.
If you're looking for 100% stability, stay with Windows XP or another OS. Not all of the hardware has up to date vista drivers yet so in theory, they might not be that stable.