I always thought the fat was one of the primary characteristics in buttermilk. The carton says it is buttermilk made with skim milk and bacterial culture. I haven't looked at a carton of full fat buttermilk lately- I don't think it was called full fat, it's just 'buttermilk.'
The fat free is definitely thicker. The pancake batter turned out dry and heavy. The flavour wasn't the same, and they were a little rubbery. Some of those details could be related to mixing, because we're definitely mixing more because there's less 'liquid'. I'm adding about 1/4C - 1/2C more buttermilk to the muffin batter. Without it, it's more like a big ball of dough than muffin batter. In this picture you can see the difference between the apricot buttermilk muffins and the blueberry buttermilk muffins. The apricot muffins are a little rubbery. They'll be hard to get through.
I'll probably continue to buy the fat free since it's plentiful around here, but I'll be using a lot more of it to keep the recipes edible. Has anyone else noticed a difference?