I only ever bought one Apple computer new, an Apple iMac DVSE (well two if you include the iPod Touch!). Since then I have had a series of second hand Apple laptops, all of which still work. I suppose anything second hand is a gamble, the trade off if is this:
Buy new : More expensive and actually more likely to develop a fault, but you get waranty cover.
Buy second hand : Cheaper, and by this point, if it is still working, it is actually less likely to develop a fault due to a manufacturing defect. You are likely to experience age related failures though, but this is typically of easily replaced parts like hard drives, optical drives and fans.