OK, before I replied I checked my account. I have $77.90 saved up. I've been a member for years. So...
Will you save up much money? No.
Will you put a kid through college on it? No Way. (I've got 14 years to go, so maybe I can buy a book)
BUT... Is it worth it? To me, Yes.
The reason it's worth it is because there is no work involved at all (after the initial start up) You just sign up and link your credit card and then you can forget about it.
I just looked at my account (first time in a long long time) and in the 120 day acct. statement I've earned money for cc purchases at mobil/exxon gas, bed bath and beyond, gnc and 2 local restaurants.
The restaurants are the best deal by far. I have 3 local restaurants that I know about (and dine at) so far (a pizza chain, a mexican chain, and show-me's bar and grill (great chicken wings!) They all pay you 10% cash back. The best part is when you get an email out of them blue from upromise saying "congrats, you just earned 2.90 from...." It's a nice little surprise.
I do have a 529 plan started for my children, but it's not through one that is available on the upromise site. And you won't earn enough to start one through the site alone, so before I joined I called and asked about it. And the bottom line is that the money is yours. You can call at any time (probably a min. dollar amount though) and say send me my money and they will cut a check to you and mail it. You can then invest or spend the money any way you like. It's really just like any other reward program, but they offer the 529 plans, it's their selling point.
Another great feature (I've never been able to do this though) is that you can sign up family and friends credit cards and link them to your account. It doesn't cost you anything, it doesn't cost them anything and everybody still earns what ever rewards their credit cards may be offering. But if you have more understanding or open minded people in your life than I do you can get them to do it (actually you can do it for them if they trust you with thier cc number) My mom (who uses her CC for everthing and pays if off each month like we do) won't let me link to hers because she "has 11 grand kids and it wouldn't be fair to the other 9 if my kids got the free money!"
OK last point...If you remember and you want to log in for you online shopping you can save even more. In fact they are one of the best to go through for disneystore.com purchases, they used to pay 6%, but now it looks like 4%, but that's still better than mypoints. That part is a little work, but if you don't mind doing reward things, it's worth it.