Just my experiences....
I have a lot of Macs... Don't want to think about how much money I have in them for me, my Wife and Children and my work. But I only have 3 PPC Macs, my upgraded G4 Sawtooth (because I really think it is a piece of art with the matching lcd) and my G5 Quads. All of the others are intel and they are great also for everything new and to keep up with the new software and and OS's.
But back to my G5 Quad... I run Leopard and it runs it very nicely (until recently I was still running the phenomenal 10.4). But my G5 (and this is not an exaggeration) heats my entire office if I have it on for a while. I am running it with a flashed 7800 GTX and 16GB RAM. I do a lot of 3D work with Cheetah 3D for my Mac game development on my Quad. I even created most of the content for my first Mac App Store racing game on that G5! But it really does create quite a bit of heat if used heavily.
My Mac Pros that are set up with my racing setups do not create nearly as much heat even when running games for long periods of time. But boy oh boy, when I run them together with the PS3, I can heat the entire office and more in the middle of winter!
Just a sad point (maybe, maybe not?), I just cannot let my PPC Macs go. There is something psychological about them being non-intel that keeps me using them. I have nothing against the intel Macs, but I really enjoy working with PPC. It is definitely some sort of mental hangup for me and the G5 Quad, even with all of its problems, is the absolute pinnacle of PPC for me.