Everyone who is against staying at a Walmart should try it at least once so they know what they are talking about. If you have, good on ya. If you haven't, shame on ya.
I find it exciting after a day's drive to putz around the store, go for a walk in the local neighborhood and most of all see what other Walmart campers have and do in terms of camping. We have met all kinds.
I highly recommend a one-nighter now and then. It adds to the experience.
It's not about price, it's about doing something a bit differently as well as not having to be tied to a specific place at a specific time. There is a great Walmart Yahoo group that has developed a highly sophisticated spreadsheet on the stores that allow and don't overnight parking including their GPS locations, store hours, and more.
Worth looking into.