In my workplace (c. 750 desks) we have standardised on Windows XP for non-technical users.
Technical users can use any OS they like as long as they support it themselves and they sort out the licensing. Quite a few tried Vista, but I think they're all back on XP now.
We find that there are a bunch of advantages in standardising and very few disadvantages, but perhaps I'm missing something and we'd be better off with a large number of Macs and Linux users and four or five Windows versions.
If you are standardised, what OS have you standardised on - and how big are you; I expect that the experience of a 10,000 desk organisation on NT4 will be very different from a 20 desk organisation on Mac OS X 10.5.