I mine with both 5850's and 5870's and think they're definitely a great choice.  For bang vs. buck they're probably the best choice.  I am running 36 of them myself.  

I did buy 15x 7970's a short while ago and just had problem after problem with them and ended up returning them to the manufacturer for a refund.

One thing to consider is that even identical cards may yield different results in terms of hash rates and temperatures. Some of my 5870's hash at 440kh/s at around 60 degrees, while others run at 80 degrees hashing at 380kh/s.

Same goes for the 5850's.  A few of them I have to downclock to 280kh/s to keep temps around 80 degrees while others run close to 400kh/s and I can't get them to run above 60 degrees no matter how hard I push them.  

You would think cards coming off the same assembly line would be identical to each other in terms of operation, but that's just not the case.  I guess as long as the manufacturer feels they meet a certain benchmark, they're ok to go.

Either way, go for it.  At 100 bucks a piece, that's not a bad deal.  I get mine new directly from a manufacturer in China and pay only slightly more than that.