The funny thing about the G5s is they are at first glance built really well. There's not a single gummy capacitor (the kind that like to blow after a few years) to be found ANYWHERE. It seems that the majority of failures either come from the power supply, thermal sensors, or the memory controller failing.
I can't entirely figure out why either. Most of them have 600W ACBel power supplies - I had an AcBel in my Quicksilver G4 and that machine ran 24/7 for about 6 years (with a couple of 2 day breaks for maintenance). I had a Vaio with an AcBel power supply in it as well, and that machine still works (13-14 years later!) It never ran 24/7 but I really used it.
Maybe Apple's engineers made a bad choice when they decided to run the fans so slow in the machine.
I'm using a working G5 at the moment - a 2nd gen Dual 2GHz. I'm convinced one of the thermal sensors is going to fail someday, but it runs fine as it is. Truth is, I feel kinda sorry to part out and sell a machine like this. XD
Much against my better judgement, too >>
Meanwhile, I feel worse about my MDD G4 - I bought it NEW 2 years ago, and although the HD I have in it right now is a piece of junk it runs fine. Problem that loud shill WAAAAAAAAAAAAAAAHHHHHHH noise it makes. A week of constant use and I want to go at it office space style.
So I'm not sure. Maybe I'll part the G5 out, sell the G4 for some cash, sell some other odds and ends and hopefully have enough to afford a Mac Mini? But then I'd have to sell my ADC monitors and get something that runs off it! Crap!
The power supplies nuking out is what gets me though. I've never heard of a G4 power supply failing but it seems as if the G5 power supply failure rate is as high as 50% after 5-7 years. Yikes!
That said, I'm going to say Apple should've considered that a usable lifetime for a tower like this is about 10-12 years. No joke. I still see a lot of PowerMac G4s in operation in places like my college! Come to think of it, they have 2 labs full of G5s that are still in use too. XD