Since Blizzard started making games in the early '90's. Since MacPlay started porting PC games for Mac in 1990.
I got my first Mac in college back in 1995, it was a Power Mac 6400. Two of my roommates at the time owned PC's... one was a Pentium 166 and the other a Pentium 200 (top of the line at the time). My 6400 came with a software bundle in the box and one of those pieces of software was the game Descent (pretty big game at the time). We had played Descent on the two PC's on a regular basis and loved it. When I popped that "MacPlay" version of Descent in my new Power Mac for the first time everyone in the room who were initially scoffing at the fact I just got a new Mac instead of Pentium instantly were silenced due to my 6400's vastly superior graphics, sound, and much better frame rate.
The same thing happened the following year with Command and Conquer. First release on the PC followed by a Mac release. The performance of my 6400 vs the PC's was ridiculous. The PC's would always bog down when a lot was happening on screen, my Mac NEVER did.
I say ALL of that because you asked "When have Macs EVER been good for gaming?" As far as I'm concerned the answer is since ALWAYS.