I've heard of it. Some of my friends are participating actively and they claim to feel the difference, being more grateful and having a log of photos to hold on to.
I'm not sure whether it really helps in the long run though, or whether it eventually makes individuals cling on too tightly and fight too hard to "be happy" when we should all "just be" and "just live".
I'm a tad torn about this.
However, I do have an app called Happier where I upload positive moments every other week. Curious to know what are your thoughts about it Xarah? :)