I'll probably read this later and say "what the heck was I thinking", but at the time (0430 Saturday morning), I felt I should say something.
The performance is terrible on my G4 iMac. Slow, slow, slow, at least with Ubuntu 7.04. 7.10 and 8.04 cause other problems with the iMac that I'll not get into here.
The biggest problem (I believe) is driver support. The video support works in just the most basic of fashions. None of the 3d acceleration works, so all of the eyecandy that's available doesn't work. The power management for the display doesn't work, so you leave the backlight on the LCD all the time. The WOL functionality doesn't work. A bunch of other stuff that, in the grand scheme, isn't that big of a deal, but put together, just annoyed the heck out of me.
So, since all of the G4 LCD iMacs shipped with OSX, I decided to reinstall it on this machine. It works fairly well, with some caveats:
10.3 (Tiger) has its own share of problems, but it is what shipped with this machine, and since I'm cheap, I'm not going to shell for 10.5. Yet. It's the last PowerPC version going to be manufactured, so it may come way down in price if I wait until the next version. That being said, I have worked through most of the stupidity of Tiger, and am fairly happy with the finished result.
The monitor now goes to sleep as expected. I have the openVPN server running on the machine again using an older version of tunnelblick which has a compiled version of openVPN included. Apache (1.x, unfortunately. I think I can upgrade it, but I'm not too worried right now) is running on the machine. OpenSSH server is running. Synergy for desktop control. Windows shares are talking. Everything that I had on the Linux box, I have on OSX 10.3. Older versions of most of it, yes, but functional versions. And the desktop is much more responsive.
The biggest problem with 10.3 is that most of the applications that I want to use are currently complied for 10.4 and later. Apparently there was some sort of major architectural change that happened at 10.4 that I'm not completely clear on. Something major, however, as most everything that I wanted to use I had to download previous versions.
Perhaps in the future I'll give Linux another swing on the iMac. Right now, however... I can see the allure of OSX. A lot of the power of the UNIX (BSD, techically) OS with the polish of a great MacOS UI.