Unbelievable for tool with such set of functions, but Its free. :)
It's extendable (export templates, collections of elements).
It supports different types of data (interfaces, flowcharts, regular charts).
I can create dynamic prototypes with it.
It supports bootstrap styles (and a few more styles for the moment, but its free library is growing constantly) of elements (with additional library) which adds opportunity to create high fidelity wireframes.
Basically it has almost everything what UI/UX designer needs in their everyday work.
It also gets my bonus points for mobile (android & iOs) stensils.
I like the fact it's cross-platform: even though I'm completely Windows user, I like that I have a choice: if, on some point of my life, I'll move to MacOS, my previously created on Windows Pencil projects will stay available for editing. Review collected by and hosted on G2.com.