It’s sort of worth it. I usually think of my groceries as a weekly thing, and if the store is presenting an option like 2 for 1 chips, I ask myself if I really need to consume two bags of chips this week, since I know if I have them, I’ll eat them. Yes, the cost per chip is down, but the deal will double my chip consumption, which I realize I don’t actually want to do.
If it’s a discount on batteries, baby wipes, bandages, or some other essential, or on something I can stash in the freezer for a while, I’ll buy for the discount.