It's lite, it's simple and you can start using it very quickly. No need for thinking. The UX is fantastic and it's very intuitive. I was looking for a tool like this for months and it does everything I need (very simple, black and white to the bone wireframes)!! If you're a lite user, you'll love it.
Lite, easy to use and quick to get started
May be too limit for some (but then you should get sketch anyways)