I was in the same boat as your friend. I'm a PC user that wanted the Air for its hardware specs.
I've had the MBA for about 2 months now and I absolutely love it. I ONLY run Vista. The two or three times I've booted into OSX I've found it to be slower and to run hotter than Vista. I mostly use it for web browsing but I also do a fair amount of word processing, run some of my work applications, and sync my phone/pda. I have Vista Ultimate and Office 2007 installed and it runs like a champ. The only application that runs at all slow is iTunes. I don't run Photoshop (CS3) often, but when I do it works fine. The only time I notice it getting hot is if I'm using the laptop for more than an hour in "High Performance" mode.
A few things worth noting:
If you're only doing web browsing, set your power mode to "balanced" instead of high performance. This keeps it cooler.
The touchpad works fine for scrolling and right-clicking. You will not have gestures or tap to click.
It does a better job at waking from sleep than any notebook I've ever had. From sleep to awake+online is 8 seconds.
I have no problems watching SD or HD video content in iTunes.
You'll need to get some screen grab software because as far as I can tell there isn't any other way to take a screen shot.