I've bought two refurb Macs over the years from the online Apple Store. One was a Mac Mini, which I had absolutely no problems with, and got bonus RAM as well. My iMac was a different story, though. During the first year, I only ran into some overheating problems running games under Boot Camp. I though nothing of it, and simply played the games on my Windows PC. Things came to a head when I installed Mountain Lion, though. Mountain Lion pushes the GPU much harder than Snow Leopard, and my iMac kept crashing, despite blowing out all the dust. It appears that the actual problem was a defective GPU heat pipe, which has apparently been bad since Apple shipped it to me.
I'd buy a refurb from Apple again, but next time I'll make darned sure I give it a proper burn-in. Speaking of which, is there a stress test for OS X?