I did that with an iMac about 2 weeks ago, and it has a faulty harddrive, which is probably going to fail soon, there's dust, or something like that in the screen (although not very visible), and the brightness on the screen flickers from time to time. Apple is sending me a new one (or at least a different refurb one), but I'll have to ship mine back, and be without a computer for a few days.
I'm not saying buying refurbished isn't legit, but you risk the hassle of having to send it back/get it repaired. Even though Apple will replace it or fix it for free, it's still time consuming and annoying.