I just wound down an engagement in which I deployed Macintosh systems, mostly iMacs, at a startup client. Except for one Vista laptop, the entire office, both business folks and Web developers, are using Macs.
The bottom line is that work is getting done and most users seem quite happy with their machines.
Nonetheless, a few copies of Windows XP running via VMWare were necessary to run such programs as Microsoft Project and Visio. This adds some expense, albeit only for a few stations. It does put into clear focus the absence of these top-notch business applications from the Mac software portfolio.
For this and other reasons, I bought a Mac Mini for my own use. One of my goals was to test the assertion, strongly implied by Apple's brilliant series of TV ads, that "Macs just work" and, of course, that "Vista is terrible." I spent five intensive days working on the Mini, during which time I encountered system crashes and the Mac's refusal to return inserted CDs. There were other small annoyances.
During that same period, I experienced no problems with my XP Pro laptop or my wife's Vista laptop.