A guy I work with had a Mac Pro Desktop, wanted to upgrade the GPU. Apple wanted to charge him twice the going rate for an ATI card that. He said screw that, and bought a HD6770 off dabs. Turned out apple used a different power connector and he had to buy a special adapter off ebay to power it. After all that it still wouldn't work because he had to flash the firmware or something. He got horrible annoyed, gave up and sold me the HD6770 for about 75% what he paid. I happily stuck it in my 4 year old machine and it ran without issue.
Macs "just work" as long as you're prepared to pay apple lots of money to fix things you could sort out yourself on a PC