I have a friend you opened one of these for her kid. As long as you're comfortable with buying the products offered and the companies that participate, you'll quickly build up money, abeit very slowly (5, 10 cents at a time; it's quite small).
Since we eat organic/fresh/non-processed food and none of our companies are in the U-promise chain, it wouldn't work for us. I'm also not interested in helping big corporations get even bigger, or allowing them to know my buying habits. That is something else you'll want to consider.
I believe you can make some money doing it, but I think saving money in a special college account and investing wisely will get you to where you want to go even faster. We have a college fund set up for my daughter through Thrivent, and she has more in there from just putting $50/month into her account than my friend has in her son's U-Promise account that she's been using for 10 years.
Just depends on how you want to accumulate money/college funds.