The key here is "family of four". I can't see that $70 a month for a single phone is going to be a big draw for a one-off phone user.
Definitely. And for me (who uses 3-4GB/month), along with my wife and mother-in-law (who use virtually no data) on a combined plan, it isn't a compelling deal. However, when my kids are a little older and get phones, it'll probably be the best thing going.