I haven't seen any negative press from G5 users regarding Logic 9 ( at least those you have installed it!)
Why have you not update Logic 8 to 8.0.2? IMHO it is a better update and fixes some bugs from 8.0.0...
One requirement for Logic 9 no one is able to avoid is OSX Leopard 10.5.7
Also, from other reports it seems that many users have successfully installed Logic 9 on their systems and are still able to use Logic 8 too.
But, as Apple don't officially support PPC architecture for Logic 9, if anything was to go wrong you probably would not get any support direct from Apple at all. Another thing to consider is nobody knows if future updates for Logic 9. e.g. 9.0.1 will break Logic 9 on PPC.
If you're still keen to install Logic 9 on your G5 then do either clone your system and/or install 9 on a test machine. You know what I'm talking about here, Roberto... File Management ;-)
What are other users thoughts?