I'd be the first to step up and buy OS4.x for a powermac G5. I was a huge advocate for it on the Pegasos II, even offered them 10k to port it and was brutally ridiculed over at amigaworld.net if anyone remembers. That being said I'd much prefer a new system with warranty and full support as well. Its a tough call here. I personally like Aeon and admire what they have accomplished. What I don't like is Hyperion and the snails pace at which they work.
