I've only used Peapod during a period when I was on crutches and couldn't easily do a full grocery shopping. My impression though is that they're a relatively low profile service that works in partnership with grocery chains. They charge for delivery.
They were OK for my needs when I was using them. Invariably they didn't have everything in stock and didn't have the breadth of selection you have in the store. (And, of course, you can't examine the produce, etc. before buying.) For my needs at the time, they were worth it but under normal circumstances it's easier and cheaper just to swing by the store a few miles down the road.