I do most of my work on a 13" macbook.
It's not a high end mac; it's actually quite close to being the bottom of the spectrum of this years models. But its specs matched the Windows machine I was working on at the time, and the reason I was buying it was for iPhone Development. A friend of mine had recently built an iPhone app on lesser specs, so I figured that the cheap bet was the safe way to go for doing indie development.
Shortly after I got my Mac, I used BootCamp to install windows on it. I was surprised by how wonderfully Windows ran on the mac. The aforementioned Windows machine was literally a piece of junk ... typing into firefox's URL bar often resulted in a bit of lag before the text appeared. This was partially due to the machine being used for over a year without a lot of cleanup, but it was still painful to use as a development machine.
On the mac book, when I clicked 'My Computer', a window popped up named 'My Computer' in less than a tenth of a second, as opposed to the Dell, where it usually takes 3-4 seconds. It was, quite honestly, for a geek like me - a breathtaking experience. I was running Windows...and it worked. WELL.
Even Visual Studio loads in under five seconds, where on the Dell it could take upwards of two minutes, without any project being loaded on it.
(This is, of course, the spoiled part of our age - that two minutes can seem like a long time to wait for something to happen).
There are two downsides to the Mac I've since uncovered though. One is that any game I am developing runs at choppy frame rates that I can't reproduce in other places (the games get roughly 60 fps on the Dell). I don't know if this is a graphics card problem, or a driver problem, or a 'your code is inconsistent' problem, but it does make working on games a little more problematic.
The other problem is that for the past week, with little to no warning, Windows has occasionally BlueScreened because of a driver failure, then shut itself down. When this happens, it will usually happen again shortly after starting up again.
Windows did recently update, and it hasn't happened since, so I am hoping that perhaps the update fixed it ... but we shall see.
So; Windows on a Mac: Mixed Bag. Windows runs way better than it has on any machine I've used recently (including a clean install of Vista on an HP TouchSmart), and I am still spending more time being productive, even with the Blue Screens. But the slowdown when running games is a bit of a pain.