E.T.
My best friend signed up when her first daughter was born and it was slightly offputting because she had to give Upromise all of her family and friends info so that we could all contribute through our purchases. A lot of friends were offended (because they wanted to save for their own kids, not hers), and most of her extended family didn't even bother to sign up. I *still* get emails from Upromise, even though I've tried to unsubscribe. Anyway, my friend stopped using it after a year or two because the money just wasn't adding up. She was using them as a way to initiate online purchases, but that meant she couldn't get some of the coupons from the online retailers like if she'd have gone direct.
I don't think it's a scam, but I also don't think it has much value for the work you will put in.