Someone made a comment on another thread that Vista has gotten the nickname "ME2". And while that may be oversimplifying things, the truth is that in the big picture Vista is likely to be remembered more as a disaster than a success. Maybe "New Coke" would be a better analogy.
In my rather limited empirical experience I am seeing a lot of people purchase ill advised Vista upgrades, only to see them dump them in favor of their old XP installation. Not techies, mind you, but regular end users. End users who, to put it bluntly, do NOT like the new O/S.
I have said for months that Vista may be the O/S that pushes Linux into the mainstream. I honestly like Vista, but when I put myself into the seat of someone who is not very familiar with computers, it's a pain. Many people have spent time painstakingly learning the basics for their XP systems; by changing the file structure and even the names of the tabs so thorougly, Microsoft has put them back to square one, and made not only their computers, but their operating systems obsolete.
But the biggest users are usually the business users. And Vista is, in my opinion, destined for modest success at best in that arena. If I were managing a network of computers on XP, my advice would be simple: don't upgrade. XP's extended support will go through 2011, and MS' next OS release will be two years on the market by then (ok, given that MS has NEVER met the deadline on an OS release, let's say one year).
Windows Vista is, in my opinion, not worth the cost of upgrading. Not unless it comes installed on a purchased machine. And it may well be a significant marketing blunder on the part of the boys in Redmond.