My dad was just asking me the same question this evening. If you have a fair amount of PPC software, or predict that you will - then go for it! If the G5 is in good condition, this is an excellent price. In my dad's case, he has no Mac software, would hope for Snow Leopard and be looking for current programs, so I recommended that he hold out for an Intel Mac instead.

As a Mac collector with a lot of software and peripherals, I like to have a box or two representative of certain eras, for the sake of compatibility. I need to have some 68k, Nubus, PCI, old-world, new-world. Just the same way as I strive to keep running the fastest reliable OS 9 machines (my MDD G4 and TiBook), I think it is important to run a box which is solid for a frozen PPC X system. Then you can set it up just the way you like, make lots of backups, and always have such a system as long as you need to. Even a few choice PowerPC plugins for After Effects or VST instruments can make all the difference of this being a productive machine for somebody.

As for what to look out for? I suppose that somebody who already has a G5 should address that question. I'd say obvious signs of wear or abuse, cooling problems, missing parts.

Good luck!