I opted out of Vista with my new Dell laptop, but I retain the certificate if I ever wanted to make the gamble.
I’ve never had any problems with XP Professional.
If you bought the SKU for Vista with downgrade rights when you got the PC or ordered it with XP you’re fine running XP on that box. If you just got a straight Vista license you have to have a full retail license for XP to legally run it on that box. MS pulled the standard “buy the new software and run an older one under that license” policy with Vista. And they’re making the downgrade rights a timed thing, to expire in X years, currently 3, but I feel sure that they feel free to shorten that without notice. It's also not legal to take a license for XP you got with an old PC (unless you bought full retail XP in the box) and move it to a new PC under MS licensing terms. I'm at the end of a large Software Asset Management project and, believe me, I'm buried in this sort of legal minutae. It may not make sense, but if you're not careful you personally can be open to big fines and, if this is your company PC it could have criminal implication (at the extremes) for you corporate officers.
Microsoft is also taking away the right of their OEMs to sell XP licenses with their PCs. Originally it was going to be January, but they’ve pushed that back to June. At that point we have to stop buying PCs with XP licenses and start buying PCs with a Vista licenses, but we can choose the “downgrade rights” version and load our XP image. That's a special part number. That's what I meant by "SKU."
The big kicker is that XP goes on “extended support” in April of ‘09, which means that only security patches will be available for free after that. MS is making noises about lots of things that we would probably consider critical, both security and non-security, being unavailable without big bucks because of how they classify them. We ran into a situation like this after Win 2K went on extended support and this year's daylight savings time change was exposed as a problem. That fix was drop dead critical to lots of corporate software including legal issues (e.g. SOX compliance) and MS wanted $50,000 for that patch from us. We negotiated hard and got it down big time but we also gave them some things in our EA renewal negotiations that we didn't want to. It was ugly.
Most of my peers in corporate IT at my level (Fortune 100) are skeptical that they’ll get away with it, in the long run. I’m cynical enough to think it’s possible, even if not likely, therefore I have to plan as if it's a sure thing and we'll be forced to switch to Vista and sooner rather than later.
I’m running what looks like a year long project (at least, probably closser to 18 months) to examine Vista and plan any necessary migration. I “own” over 30,000 PCs worldwide and it looks like it’s going to be a real pain. In my industry the norm is to install something and never upgrade it until it breaks or we're forced kicking and screaming, including software. When I tell one of our business units that they have to upgrade some big business critical IT system ($$$$$) because it's not compatible with Vista I'm gonna get shot.
We won't go back and put Vista on existing PCs, but at some point we may start buying new PCs with Vista as we replace PCs in our normal lifecycle. If something comes up that forces us to move in a hurry to Vista company wide (e.g. SAP only supports running their client on Vista for some reason) we'll do a bulldozer and replace old PCs, no matter how new, with new ones running Vista, configured "correctly" (e.g. drop dead minimum 1 gig, probably 2, at least something like an Athlon 64 if not a dual core CPU from AMD or Intel rather than our current standard Intel Celeron with 512 MB).
I’m running Vista on a company PC (Dell GX755) at home as my main personal system right now to get more familiar with it and it’s really a pain. I’m actually used to and like some of the user interface changes and like some of the new features. I’m a sucker for things like the Sidebar and I love widgets. Those things actually fit the way some of our mill operations are run and could be useful, especially being native integrated part of the OS. The problem for me is that it’s a hog and that it’s "not real stable." I’m actually getting set to reformat and reload it on this PC (in the process going from a RAID 1 mirror on two 250 gig disks to RAID 0... woohoo! Enough disk space for, what, two weeks?).
This PC is a "show off" version of our upcoming standard model as our old standard goes end of life, which is still slated to be a Celeron box. In addition to the big honkin’ drives on this test system it has an Intel quad core Q6600 at 2.4 GHz, 4 gig RAM (I’m using 32 bit Vista so that translates to 3316 “usable”), one 22” wide and one 19” ~square display in extended desktop. Vista runs fast enough on this hardware. <g>
Part of the project is deciding IF we move and what alternative moves might be if XP becomes a problem after going on extended support. That means I'm also going to get to play with Linux desktop systems and... shudder... Macs. I came across this interesting article (from PC Mag, of course) referencing the new Mac vs PC ads with the annoying guy from Die Hard 4: Leopard Is the New Vista, and It's Pissing Me Off. It's not what you think. He does say that Leapord isn't all that and shouldn't be dissing Vista until they solve their own problems, but he likes Tiger as "better than both of them."
You also said:
I’ve never had any problems with XP Professional.
With over 30,000 PCs to worry about you can imagine what I think of that statement <g>