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.
I don't like the fact that during exporting project pages, it puts them on 1 large html page. I would prefer to see them on different pages. Axure does it great - with list of pages in left frame, and active page on right hand side.
Also, the same thing I miss in Balsamiq - lack of multi-level pages management. I would like to keep multi-level structure of project pages.
I would like more active development of this product (seems like it's sort of abandoned :().
I use this tool for my clients' projects only since organization I'm working in prefers using Balsamiq.
I like an opportunity to show my clients almost live website where they can basically feel it. It's great that we have such tool for free!