I was a Kickstarter backer of this project back in October, and it kept me on track during NaNoWriMo. It's easy to use, has never caused any installation problems, and it's a real kick in the pants when I look at my website and realize I'm behind.
I love "Get Book Updates" feature that will add the people who click it to your mailing list. I also love that pro users can have more than one book, since I usually have one novel I'm drafting and another I'm finishing up.
EDIT: How could I forget? When you update your progress, your dashboard has fireworks. How cool is that? Such an encouraging feature. :)