Microsoft sent over a copy of Vista Ultimate and I upgraded my Media Center test box to put it through its paces. I have had mixed results.
First, the positive. Vista is building on XP Media Center, which was already a good media platform. XP Media Center 2005 Edition crossed over the threshold of "good enough" to serve as a PVR instead of a TiVo or ReplayTV. I found it quite stable, though it still needs an antivirus subscription, and works best as a DVR when used almost exclusively for TV rather than combination work/TV/test box with all the software detrius left by constant loading/unloading of drivers and 
programs. Vista Home Premium or Ultimate is even more refined, with better graphics, transitions, and transparencies throughout. The main user interface screen for Media Center now forms a cross (up/down to switch media, left/right to move through options); this makes the various functions more accessible at the cost of some simplicity.
The OS as a whole has improved in many small but meaningful ways - networking and file management have gotten particularly useful overhauls. Overall, Vista looks and feels a lot more like a Mac, only without as much of a learning curve for people used to Windows XP*.
I had absolutely no problems with the upgrade process itself. Vista Upgrade Advisor said everything should work just fine other than a Dymo label printer which needs updated drivers. I can live without that for a while, so we were all set. I chose to wipe out all existing data and start fresh - I didn't want any old software causing problems down the road. Still, Windows insisted on saving all the old files, moving them to a separate folder for safekeeping (figuring out how to delete them en masse - they were seriously clogging up the hard drive - was no picnic).
Now, the bad news. As an upgrade, on my machine at least, it's quite buggy.
The first thing I noticed is that certain applications within Vista seem to remap the speakers. Quicken and some - not all - downloaded videos play the center channel sound through the left rear speaker. This is bizarre. Other applications play things just fine: Media Center, no problem. iTunes, no problem. Rhapsody, no problem. I couldn't find anything wrong with the driver or the sound in Control Panel, and the applications that most need sound seemed to work, so I chalked it up to version 1.0 gremlins. Hopefully a future driver update will fix things down the road.
And then the real trouble started. The system would mysteriously, and consistently, crash. At first I thought it might have something to do with Rhapsody after all, as it was playing in the background during most crashes. Eventually I discovered what I suspect is the real problem: the Ribbons screen saver. About 20 minutes in, it takes down the whole system, even if its the only thing running on the PC. Well, it's probably not the screensaver's fault, but my video card. Running the diagnostics program within Vista tells me everything's fine, but 20 minutes of that screen saver and crash. Every time. In its automated search for a solution to the problem, Windows has since told me that some part of my video card driver is incompatible with Vista, and that I should check for updates. I checked. Nothing new. However, with the screensaver off, the system has been completely stable. I suspect - hope! - that updated drivers for the video card will be released that solve the problem, not just avoid it.
Unfortunately, with the screen saver off, the system is stable long enough to go into sleep mode. That's perfectly normal and saves power - and money - while hurting the environment a bit less. The problem is when it wakes up from sleep mode, most of the time my Logitech DiNovo keyboard and mouse just don't work.
Needless to say, this makes the whole system useless - you need a keyboard to type in a mandatory password to unlock Vista before you can run the Logitech software to reconnect things. At first, I thought that this might be a Bluetooth incompatibility - Logitech uses different Bluetooth drivers for Vista than it did on XP. But I checked, I'm using the correct drivers. No, it seems that this one is a known bug (KB 929577 "This problem may occur on a system that supports selective suspend if the computer goes to sleep shortly after the Bluetooth device's power is cycled.") with a patch available. Great, they know about it, they have a fix available, I should be good to go. No such luck. The fix is not actually available for download yet. You can ask Microsoft to send it to you, but when I tried that using the online chat system, it got as far as telling me that there would be no charge to answer the question, and then... "An unknown application error occurred. Please try again in a few minutes." A few minutes, a few hours ...has made no difference, I can't even ask the question. Yes, I could try emailing or calling support on the phone (or calling Microsoft PR), but for now, I give up. I'll email later, and in the meantime I've changed the power settings so that the computer never enters sleep mode.
These are just my experiences on my specific hardware. Generalizing from my experience is not necessarily fair to Microsoft, and I would not be concerned about buying a new PC with Vista preloaded (in fact, I plan to do that soon). Having said that, based on my experience, I cannot recommend upgrading an existing machine from XP to Vista at this time.
-avi
*Oh, I know I'm going to get flamed for this, so let me explain: I hadn't used a Mac in about four years when Apple sent over a MacBook Pro running OS X a few months ago. No question about it, OS X is a wonderful OS, and Microsoft clearly spent a lot of time studying it when putting together Vista. But the Mac has different ways of doing things, different places where downloaded files are stored, and different user interface conventions. For example, when closing an application in Windows, it closes when you click on the "X." That same action in OS X seems to minimize it. Right click on something in XP or Vista, and you get a context-sensitive list of options. There's a way of doing that on the Mac, too, just no second mouse button for it. Some of these are such seriously ingrained habits that it took me a while to figure out why I was confused. Anyone who wants to migrate from XP to OS X rather than Vista isn't going to get an argument from me - please, Apple fanatics, I love you! I do! - but I would recommend that switchers buy a book or have a Mac user walk you through some of the basics.