Not a great experience for me...
I bought my refurb. 24 inch iMac in April 2008. So far I have had the logic board replaced and it is in for a new Hard-Drive right now. I suppose these problems could also happen to someone with a new machine, but this iMac has just been generally unstable. There are also 3 small 'dings' around the display, where the glass and aluminum meet. No dice on getting that fixed/replaced because it's cosmetic (that's what the apple rep told me). This was my first refurbished unit and it has really left a bad taste in my mouth. At lease it came with the new mac smell... I'll probably stick to new from now on.
On the other hand, two of my sisters both bought refurbished MacBooks last year, and they have never had a problem.