It all depends on what's been refurbished and who refurbishes them. If it's a laser replacement, or something that's been cleaned/dusted or something simple like that, those are fine... however if it's some sort of fan-mod or something that "fixes" the YLOD (Similar to the 360's RROD), then it's iffy. Mainly because the YLOD may have to do with a overheating issue, and there's no real permanent fix for that, only a temporary one.
Also, if you're in the market for a fat PS3, make sure it's the ones with 4 USBs on the front. The ones with 2 don't play PS2 games.