I'm writing this on a 2.4@3.2 GHz Quad-core, 2 GB RAM, 8800 GT, 500 GB HD hackintosh. I'm guessing the components costs in the neighbourhood of $1200 in the US and, according to geekbench and xbench, it performs slightly better than the mac pro (1x2.6 GHz quad core extreme + 8800 GT) model at $2600.
It's fully updated and seems pretty stable, but Safari and Mail occasionally quits unexpectedly (like once every couple of days). However, updates are not always trivial to install. Updating to 10.5.3 will likely need some additional patching, or a bit of terminal magic.
I didn't buy this computer specifically to run OS X, but I wanted to install it to see what all the fuss was about and as it turned out, OS X has become my primary operating system, except for games (I was using Linux+Vista before).
The copyright laws in my country afford me the right to patch away at any legally bought software, if it doesn't work on my hardware, so it would seem I'm in the clear, but US law is very different. At least one of the patches circumvents encryption, which I'm guessing would get you in trouble with the digital milleneum act.
Morally, I think there is a slight difference between modifying a legally bought copy of OS X privately, and doing it commercially. If you want a hackintosh, take the time to research and build it yourself. If you don't have the time, knowledge and inclination, your best bet is still to buy from Apple.
So is Apple losing money on this? Well, I bought an Apple keyboard and Leopard and am currently thinking about buying a MacBook. That would have never happened if I did not have the option of trying out OS X relatively cheaply.