Yeah I've used both. They make hardened Ibooks that you can drop :-)
The cool part about OS X is that it IS Unix based and you can pull up a regular ol' bash shell and do most of the things you're used to doing in Linux/Unix. The GUI is very smooth and responsive and doesn't have a very large learning curve if you're used to Windows/X. They are very easy to use, almost too easy, which is exactly what Apple is trying to do.

Admitadly, I'm not a "power" user of MAC's, we just have them at work so I have to support them. I find an Ibook to be more than adequate to surf the web and goof around. I've never tried to game with it though. The power book is going to cost you more, but it is beefier hardware. I would suspect with the new G5 processor being released, if you have a little patience you'll see the price on a G4 powerbook drop considerably.

Anyway, though I haven't owned a Mac of my own in five years, if someone gave one to me, I'd use the hell out of it for a workstation.

--edit. If the Ibook doesn't have a NIC, it's because they have built in wireless. Apple really want's you to buy one of their AirPorts for wireless access. As for PC based laptops, I'm a fan of Toshiba's, I have two that are like 7 years old and still run solid. I also like the Compaq Evo series, but have heard some unflattering things about the Presario series. ThinkPads are good too, and nobody ever got fired for buying IBM!