Very high IMHO. I have a 3 year-old TiBook 15" that works great. It still looks pretty good too, other than for a few paint scrapes. The display remains excellent and the keys are fine. I also have a new PB 12" that feels very solid. The screen could be a tad brighter, but that's my only complaint.
I've used a lot of different laptops over the years, including machines from Dell, Gateway, Zeos, Compaq and Toshiba (never had a Sony or an HP). I have a Thinkpad T41 right now. To me, the Thinkpads are the only other laptops that can compete with the PowerBooks on build quality. One test I use is to press down firmly on a couple of keys in the center of the keyboard. On virtually everthing besides PowerBooks and Thinkpads, the whole keypad depresses and flexes.
Case quality is another factor: the Thinkpad uses carbon fiber for part of the case, along with titanium hinges. Apple uses aluminium and also has quality hinges.
I'm speaking only of PowerBooks. To me, iBooks, while nice machines, suffer from a lot of the same build quality issues as their PC brethren.