In the used market, you would want to steer clear of the older PowerPC based models, more specifically the Powerbooks and iBooks and go with the Intel Core2Duo models. The advantage is that you'll have access to the most current OS (Snow Leopard) and the ability to run a variety of operating systems, including those from Redmond, WA.

Common failure issues on the Apple laptops are screen hinges cracking, keyboard failures and case crack, particularly the white plastic 'books.

Compared to the Windows market, I find that Apple owners, like owners of other luxury products, do tend to take better care of their "precious." You may be able to find a used one that is still under AppleCare extended warranty if the owner opted to buy it, which many do.

Unless you need it to be ultra thin, the Macbook Air, seems to be retaining it's higher price point in the used market, despite being somewhat reliant on external devices (doesn't have built in optical drive) which can get messy.