I prefer real buttermilk in recipes to the vinegar-lemon milk substitute. It was frustrating always running out of buttermilk just when I needed it most. I usually quadruple and make a gallon at a time. Once you've made your own, you can save some back to replenish your supply by making more. My husband loves an occasional cold glass of buttermilk on a hot summer day, with a little salt.
Update: A couple of reviewers have reported this did not work for them. Because it is a 'culture based' recipe, it relies on the fresh buttermilk that provides the 'culture' to be fresh enough that the 'culture' is still alive. Once the culture has died, it can no longer feed to create more of itself.