It's only really the iBooks that get truly great battery performance; 4.5 to 5 hours of battery life in normal use is fairly realistic with them. Current-generation PowerBooks tend to get something closer to 3-3.5 hours in the same usage patterns.
Performance-wise, the only thing that Mac laptops can really claim over Windows boxes - for now - is that they all have dedicated video chipsets. A lot of 12" laptop makers force you to use shared-memory graphics systems (Intel Extreme Graphics is the most notorious example), You not only have to give up some of your system RAM for the graphics to work in the first place, you lose any hope of decent 3D performance.
There's also no G5 laptop - yet. Rumour has it that we'll see one at either the WWDC conference (early June) or Apple Expo Paris (mid-September), but what specs it'll have or how well it'll work out in real life are still up in the air.
Where Mac laptops do well is in design and feature set. As far as design, they take into account lots of little things: all the rounded, smooth edges and corners mean that they don't catch on your carrying case or jab into your legs when sitting on your lap. Slot-load optical drives mean that you don't have to worry about the tray mechanism breaking or getting stuck. That sort of thing.
Features are where they really justify themselves. For example, all Mac laptops have an instant sleep/wake feature through opening and closing the lid. You rarely have to shut off the laptop completely; you just close the lid when you're done, and open the lid when you're ready to begin again. Apple also has a good Bluetooth implementation as standard on all PowerBooks, there's a dynamic backlit keyboard on the 15" and 17" PowerBooks, and they're one of the few companies to offer DVI-out video (on the PowerBooks).
Lastly, I'd note that the 17" PowerBook is the thinnest and lightest 17" laptop out there, period. It's the only 17" laptop I'd consider if I actually needed to travel frequently. Most any other is too big and/or too heavy to make it worthwhile.