What it all comes down to in the end is... can it perform what you want it to?
I still have some old ppc macs, but to be honest, anything under 1GHz will give you headaches... a G5 shouldn't have any issues.
Mike does make some points that all of us already know. Apple has effectively nuked the ppc platform. The G5 isn't gonna run into any speed problems anytime soon... the real issue is support. 10.5 is a great OS and will continue to do what it was designed to do for years... I still run 10.2 on one of my iBooks, but it clearly shows its age, and this is only 7 years later... keep in mind Leopard is already several years old.
Coupled with the fact that the problem doesn't just lie with apple... a lot of companies are phasing out support... flash support is dead with no more updates in the future, so in a few years unless flash is killed off en masse, you might run into problems playing online videos. A lot of people have mentioned the lack of a modern, up-to-date browser will be the platform's undoing... I can see this when I run 10.2, it has some old versions of camino, firefox and safari, but they clearly show their age, pages load up with all kinds of weird. There are many independent builds however that help you get around that, and as long as they are around you should be fine.
If you're adventurous and know your way around tech, I say go for it, the price seems good, and the expandability can't be matched. You'll definitely need to be quite tech proficient to keep it running well into the future though. If you don't want to worry about these things in a couple years, then you might want to consider a mac mini... which runs circles around a G5 (before you flame me... I've tested a previous gen. C2D mac mini vs a Dual Core 2Ghz G5 with 8GB of ram... yes the mac mini IS faster).
