I LOVE Subscribe & Save. I always have at least 4 things every month which is what you need in order to get 15% off (it's 5% if you have less than 4 items that month). You get free shipping with or without Prime (I don't have Prime but never pay for shipping on S&S items). You can even do S&S on add-on items.
Make sure you go check your S&S items by the 20th of the month as they start preparing orders on the 21st and at that time it's too late to cancel anything for that month. I always go in around the 18th-20th to see if the prices on the things that are coming up are good prices (they really can fluctuate a LOT, especially if you first started buying something because of a sale) and also to check my list of S&S items that aren't due to ship to see if I want to get any of them earlier and add them to this month's shipment.
I get the following items through S&S.
gluten free groceries (cookies, cereal bars, pasta, toaster pastries, cereal, licorice, mixes)
non-specialty foods (nuts, cereal, apple sauce, microwavable lunches, snack foods, tomato sauce)
skin care products
vitamins
personal care products (tissues, toilet paper, hand soap, toothpaste, dental floss)