I, too, have an iBook that dual-boots OS X and Gentoo (and since zojas's web page about setting this up was very helpful when setting this up, I should take the time here to say thanks!

). And, my GF runs OS X on her G4, a machine that I set up for her, and use regularly myself. Unlike zojas, however, I've gone away from OS X on my iBook: after using OS X most of the time, I'm now using Gentoo most of the time; when I do run OS X, I do it mainly within mac-on-linux.
It's hard to say why, exactly, since some of this comes down to personal taste. I like a minimal window manager with multiple desktops (*box), even more than the OS X dock (yes, I know I can run fluxbox with multiple desktops in OS X's X11). I like that things seem faster on the same hardware with Gentoo, across the board (this is especially noticeable with offlineimap, a python program I use a lot with my e-mail; but still, things are generally fast enough in OS X). Portage is indeed way better than fink, mainly because it doesn't make the stable - testing - unstable distinction like Debian does (yet it isn't hard to run all the *nix software I want in OS X). I don't feel like paying Apple what amounts to about 10% of the original cost of my iBook for the 10.3 upgrade (no not much money in the end, and if I were happiest using OS X most of the time, I wouldn't hesitate).
There are other little things, but in the end, two things triggered my going to Linux full-time on my iBook, and they happened around the same time. First, the 'straw' news aggregator got stable enough for me to survive without the absolutely wonderful NetNewsWire Lite in OS X. Second, I had a rare crash in OS X one day, and even rarer, checking the disk after rebooting with Apple's disk 'first aid' identified a problem that it couldn't fix, even after repeated runs, and even when I booted into single-user mode (text console only) and ran fsck from there; this, to be honest, made me lose a tiny bit of faith in HFS+, enough for me to feel more comfortable with ext3, which I know isn't perfect, but I've never had disk errors in ext2/3 that I couldn't fix with fsck (yes, I know about DiskWarrior, but I don't want to have to buy software to fix something that should be fixed by a built-in utility). Neither of these things amount to a reason to say that Linux is
better; they just seemed to tip the balance just enough for me not to think about using OS X every day.
All that being said, OS X is a fantastic OS, and I'm really impressed with how Apple fused the BSD bits with their proprietary OS X bits on top -- fire up a terminal with your favorite shell, and it's thoroughly unix-y. Besides, iTunes is great, multiple network locations are a cinch, it's just about as stable as Linux, it is pretty (in spite of the lack of customizability), it's certainly easy to use, etc. etc. etc. I was able to work in OS X just about as efficiently as I do in Linux, and things like MenuMeters even made me not miss gkrellm too much! If I were forced to use only OS X, I wouldn't be disappointed.
So, I strongly recommend OS X to anyone and everyone, unless you have a near-pathological desire to tinker with everything on your system and/or use old hardware, or want to run only free (as in speech) software as much as possible. I guess those reasons, in the end, are the clearest expression of why I stick with Linux.