Count me in: I have for 512MB sticks in my G5 from Crucial, and haven't had a single problem. I didn't find them particularly difficult to insert, either--not nearly as hard as an iBook I upgraded a while ago--you just need to get the angle right. Laying the G5 on its side helps.
I believe Apple has changed their policy regarding 3rd party parts; I'm pretty sure it used to be that if you installed anything yourself, it would void the warranty or at the very least they'd refuse to work on it, but of course since they can't tell if you take the parts (RAM, HD, etc) back out before you send it in, people would frequently do that.
At some point, though, Apple started listing a few things as user-installable parts (there's a list in the manual); generally limited to RAM and Airport cards, but the G5s probably include a 2nd SATA drive and of course PCI cards, as well. According to the brand new eMac manual I'm holding, Apple "recommends that you have an Apple-certified technician install memory", but the only warranty stipulation is if you break it while installing memory, it's not covered by the limited warranty.
So, bascially, so long as its listed in the "Enhancing Your Computer" section of the manual, you can install it yourself without affecting the warranty. Just don't break the computer while you're doing it.