I had a MacPro1,1 and ended up getting this since I moved and didn't want to lug that with me. The fact that I couldn't natively run Mountain Lion on the Mac Pro without hacking was the end of it for me, and this computer is a serious upgrade in many regards.
Still getting used to designing on Retina, but so far so good. Miss the redundancy and speed of my RAID array, but SSD isn't bad. I've killed a MacBook Air's SSD before, though, so I'm relying heavily on "the cloud" with this machine as a mirror in case something happens. (which is fine.. we didn't have this stuff when I was on the Mac Pro)
Not sure about the gaming performance with the Retina display as I had a beast of a video card in the Mac Pro, but I'm sure it's fine.