I have four. Not bad. I get a steady 220 mhash on them but I don't really push them that hard. They are more like filler cards for the 4th slot on my rigs cuz my PSUs can't handle four 5850, but they can handle three 5850 and one 6850 under clocked a little.
I think you can unlock shaders and get more performance out of them.
Thanks for the input, I probably won't push mine to hard either, it's just a fun project, and I think I'll use the excess heat from the cards to keep my hedgehog warm, she has to stay at 72 Degrees minimum.