|Don't ask to ask, just ask|
I have an old PPC PowerBook that started with Jaguar and is now Tiger (OS X). Just recently I had to replace the hard drive, and I replace the battery every 10–15 months or so (I overuse it), but otherwise it's been solid for years.
For $work, I have a Dell Latitutde (D620, it says on the front) running Ubuntu 7.04. I've had it for about six months, and it works fine.
I like Ubuntu better for programming. It has the nice package management I'm used to, so I can just ask it to install or uninstall all manner of Linux goodies. On the other hand, making the wireless do what I want seems like quite a pain. I haven't gotten WPA to work at all, and I sometimes wind up dropping to a command prompt to issue iwconfig commands to get things working. That's been a fairly typical experience—things that should be easy, aren't. I stopped trying to suspend it because it would usually wake up confused. I have never tried anything like bluetooth.
I like OS X better for desktop type stuff. It's where I have my music, where I watch movies, where I pull photos off my camera, etc. For the most part, it just works. The biggest problem I've had with it is that it is closed. If I want to do something off the wall, I can't always go out and download some half-baked patch and do it. That said, it is Unix underneath, it does have Perl, and there's always ssh.
Update: I forgot to mention, when I retire the PPC eventually, it's probably going to get a Debian GNU/Linux install and sit in a corner as a headless machine. I haven't tried using Linux as a desktop on a Mac, but I have put it on another PPC laptop whose display was destroyed in a toxic corn syrup spill. It's working as I'd expect any other Linux machine to work.