I do subscribe and save for things like tea, baby wipes, rain drops (we cloth diaper and have hard water), and litter genie refills. We also sometimes use it to buy toiletries in bulk (like 5 packs of toothpaste / soap, etc.). We tend to buy less chemically stuff in these areas (Tom's, Dr. Bronners, etc.) and Amazon is often cheaper than grocery stores for these brands. I haven't found it to be useful for general groceries, really. We got a bunch of pantry coupons and haven't used them.
I really like the idea of using them for a vacation, though. We frequently do VRBO, and having groceries delivered seems like a nice idea.