They tend to look very nice, but they have (speaking of powerbooks) uncompetitive battery life (compared to centrinos), poor screens (compared to even a normal middle of the road laptop, but particularly compared to the brightview-style laptops), and they're somewhat expensive too.
They do have non-integrated graphics cards - a plus - and they do have a very attractive desigh, so for those that care about such things, those are two plusses.
The problem with the graphics card is that they're fairly last-generation stuff - Radeon 9700 in the high end and GeForce 5200 in the low-end - and the GF5200 used in the PB is low-end.
The G4 doesn't compare to the Centrino speedwise - it's about 2/3 the speed of the Centrino, clock for clock.
Apple desktops are out-of-this-world expensive. A Mini + 19" LCD is $900 and more, and that's for last-generation hardware. A Dell 4700 + 19" LCD is $480 or so, with current generation, reasonably fast hardware.
The GUI's great. The software included with it is great. The extra software that you can buy for it, though, just doesn't compare to Windows outside of some mainstream interests.