The only thing I dislike about it is that I had making something similar on my long-term "Someday/Maybe" projects list. They beat me to the punch.
But really, this thing is insanely great.
Basically, it gives you one message at a time in a nice interface, and you can't see the rest of your messages or inbox. Super easy to clear the inbox. They also give you a timer and a score at the end of it, which pushes you to go a bit faster.
I really, really, really dig this. Highly recommended that you try it if you're on Google Apps or Gmail.