I can't speak for the unibody, but earlier powerbooks seem pretty durable, presuming you're not handling your laptop roughly. I've been using a powerbook g4 for nearly 7 years, and it still works decently. Granted, I had to replace the superdrive once (breaking even on applecare), the hard drive (by choice, not necessity), and the ram (choice, then later on, necessity when the ram chip died... also, one of the ram slots died too, known problem). The screen has developed 2 or 3 stuck pixels, and the backlighting isn't completely even (there are a few brighter spots). At low backlight intensity, the bottom corners of the screen yellow dramatically. The battery lasts about 90-120 minutes under mild use (which is probably unusually long... however, I spent a long time playing WoW where I simply popped out the battery and didn't put it through the constant charge/drain cycles).
Oh yeah, WoW... this thing has ca. 150 days /played on it, so its seen a bit of stress. It has been my primary computer since I bought it... based purely on the hours I've put on the new hd, I'd estimate the laptop had ca. 1000 days powered on, or roughly 9-10 hours/day.
My brother has the same model laptop, and he also still uses his... his hard drive died once, other than that it is equivalent to mine. Note that not all laptops will be equally reliable, not between designs nor even between individual laptops of the same design. In general, the apple computers we've bought over the years (talking LC II - present) have had good build quality, with the possible exception of the performa line.... but that's old history.