Sarah, I agree with you that REAL buttermilk is the best and I like using the low-fat. However, for a few months we had trouble getting it in the supermarkets here (Eastern PA) -- so for certain things I used the powdered (bulk from the health food store - like that a bit better than SACO) - it is ok - for certain things.

Of the substitutions you listed, I usually use the 2/3 nonfat plain yogurt + 1/3 cup milk (at our house that is skim or 1%) and it works best.

We don't mind the powdered (if necessary in baking) but in things like soups and chowders I need the liquid.

To me its interesting all the variations and substitutions one can make with so many different things ...