I assume you're talking about refurbs direct from Apple.
I've bought refurb computers from Apple for several other people, and haven't had a problem. The one that had a hardware issue (bum optical drive in a PowerBook G4) was repaired under warranty, so while a minor hassle, the same thing could just as easily happen with a brand new computer.
Besides, if you're worried, you can get AppleCare to bump the warranty to 3 years, which gives you a pretty good amount of leeway in the event something breaks.