Adding to the anecdotes[1], of the last 5 computers that I have owned, 0/2 of the Macs are still running, compared to 2/3 of the non-macs. Because of this, I refuse to ever purchase a used mac, the discount is not worth the risk, especially considering the fact that it's nearly impossible to repair them on your own.

[1]Random idea for a website/service: Computer reliability statistics by model/year. Could be really useful for people in the market for used machines. Not sure how/where you would get the data though. Seems like forums are overrun with anecdotes, but actual data is few and far between.