Unless it was less than 1/2 of a new laptop I'd pass. There are issues like the aging battery or other gremlins in all used laptops. Since it's unlikely you'll pop for Apple Care this adds to my negative thoughts on this.
My friend is offering me a good price to buy my 17" G4 Powerbook (1.67 ghz). I love Apple's OSX and plan to be a Mac user. I'm debating whether to upgrade to the Macbook Core 2 duo. My G4 does have a very good video card and it's very thin/stylish (with it's metallic case of course). But with the size of the 17inch powerbook, it's pretty heavy and takes up so much space! I like the Macbook because it's smaller, lighter, houses Intel Core 2 Duo, and makes it possible to run Windows XP. But, the Macbook does feel cheap with its plastic casing and low memory video card.
I typically use my Powerbook to do web browsing, Microsoft Office stuff, music listening, and some graphics design (not much).