This is a flame war topic so I'm going to start it by saying this:
I have only worked with PC so I don't know much about Mac except what I have heard. I love doing graphics and digital art and see no reason for switching to mac for this when it seems to me that PC offers more software and compatability. Here is some work I have done using a PC loaded with Adobe Photoshop 7:
t3ch.l33t Image Arena. I have also heard that mac is considering purchasing Intel processors for their laptops because they simply can't compete anymore due to their energy needs.
The only pro I have heard to getting a mac is that their gamma is closer to actually printing than PC but I have also heard that PC has this fixed for the most part. My printer allows me to change brightness settings and Adobe comes with a Gamma Loader that is supposed to help. I personally don't see a problem when I print and I'm a stickler about that sort of thing.
Personally I like PC because it is a heck of a lot more usefull than a mac will ever be. I can play all the latest games and most software that comes out is for the PC. Mac just sounds like a big pain in the you-know-what to me.