People successfully completing the challenge claimed to:
Start noticing what makes them happy every day;
Be in a better mood every day;
Start receiving more compliments from other people;
Realize how lucky they are to have the life they have;
Become more optimistic;
Fall in love during the challenge.
Even when the challenge is over the collected 100 happy moments can always remind you about the beauty of your life. For that, you can receive a little 100 page book with your 100 happy days at the finish line of the challenge!
You can follow me and my 100 Happy Days here if you would like to.