Based on the negative performance reviews - I almost didn't give Virtual PC a chance.
I'm a recent convert to Mac, having spent the last 14 years in PC/UNIX environments. Doing graphics and web development now, I had hoped the move to Mac would be fairly painless - and it has been, with one exception. Two programs that I use significantly are PC-only, and after months of searching, there's just simply not anything on OSX that has the comparable functionality.
And, when testing browser compatibility it's just as arrogant for Mac designers to ignore testing on the PC browsers as it is for PC designers to ignore the Mac browsers.
So, I was faced with three choices: 1) have a standalone PC for these programs, 2) have a PC but going through a KVM so I could use just one keyboard/mouse/monitor to switch between, or 3) use Virtual PC.
Number 1 was out because I just don't want a completely separate setup - and don't have the room. But I was heavily leaning towards #2 simply because it seemed "right" to have my two Windows programs running on a Windows box. Plus most of what I had read on Virtual PC made it sound like it would be unusable - it was just too slow.
(Un)fortunately, my #2 option above was proving to be a costly affair. A new PC, plus either a DVI switch or a new dual-input monitor was looking to be in the $500+ range. And there were limitations with that too - like no direct interaction between the two machines, not being able to use the Mac and PC at the same time, etc.
So, for $129 I decided Virtual PC 7 was worth the shot (I already have a Windows 2000 license from a previous system). The complete setup on my Dual 1.8 G5 (2GB Ram) took about 2 hours. And I was skeptical at best - I've used emulators before and they suck.
Wow, was I pleasantly surprised! Not only does stuff (like networking, shared folders and printing) just seem to work, but the performance is perfectly fine for what I'm doing. I can see where someone who was expecting true "Windows" top-level graphics performance would be upset, because you don't get instantaneous screen redraws all the time, and after a heavy graphics-laden screen redraw the cursor may be slow to move for a second. This could definitely make things like game-play and Photoshop problematic.
But for most programs, and if you're only running 1 or 2 at a time, the VPC responsiveness is pretty impressive - especially considering it's an emulator. It is definitely very usable - typing has no delay - and it never really feels sluggish that I've been able to tell. On average, VPC seems to take about 25%-30% of CPU resources when I'm doing something in it, but if it is sent to the background, it only takes about 1-3%. That's nice.
If your goal is to use VPC to run Windows on a Mac, you may well be disappointed. If your goal is to use VPC simply as a way to run a needed PC program on the Mac, then I think you'll be impressed.
One note. Once you've got your VPC up and running, copy your VM document somewhere as a backup. That way if you ever need to "reinstall" your Windows OS in VPC, you can simply drag this copy in for a clean "install" without having to go through the real 1 hour + OS install.