My new year's resolution is to lift my long-standing prohibition of Windows Vista, but only service pack 1 (SP1).
My experience with SP1 shows improved performance over the original. This has always been one of my big concerns. Historically, new versions of Windows consumed more resources but did not dramatically alter the performance profile of a given machine. Vista definitely does. I considered the original very bad; SP1 is tolerable or better.
Vista does demand greater resources, especially memory. The existing systems owned by many of my clients were simply not up to a Vista upgrade. Two years have passed and many of these older systems have been replaced (with XP, of course), so resources are less an issue today.
I'm convinced that user access control (UAC, the feature so effectively skewered by the Apple Mac ad featuring the bodyguard) is a non-issue, annoying in the first few weeks and virtually invisible thereafter. Microsoft reports that systems with UAC enabled are much less likely to become compromised by outside threats. I believe that, but even if Microsoft is overstating the case by a factor of two, this still makes Vista more secure than XP. Anecdotally, my wife's Vista laptop has never been hit while her previous XP system was.
In light of the forthcoming Windows 7, my new position regarding Vista might be seen as premature. However, my existing clients know that I rarely ask them to adopt the newest version of Windows until somebody else has suffered the slings and arrows of being on the cutting edge. Unless I am totally bowled over by Win7, I do not expect to approve it for my clients until late 2010. That one year lag is entirely consistent with my past practice.
Vista adoption (and surely Win7) requires care. Each system must be audited and cleared (automated tools are available for this). Your software suite must be inventoried and cleared; some upgrades may be required. Finally, peripherals like printers must be checked for Vista compatibility and the availability of drivers.
As always, do not use any "home" edition of Windows in a business setting. And consider the 64-bit versions of Vista and Win7. Both require additional work to assure compatibility, but 64-bit computing is the next performance wave and must not be ignored.