I had the 6850 for a week and hated it. Ended up buying a 6950 instead (before 7-series launch - too bad you can't find them cheap anymore). It's not a bad card but this far into 2012 it's inefficient and a little lacking in features compared to 7-series cards.
The other thing to watch out for is going to be channel availability later on this year and next. It might be very expensive to buy another 6850 to crossfire vs. definitely being able to purchase another 7770 for even cheaper than what it is priced at now.