I just used pencil in my company to create a mockup for a recent web project and it was a good experience.

PRO: It gave good results to us. With Bootstrap + FontAwesome stencils we were able to create more realistic pieces and gathered better feedback.

CONS: The linked repository version had a glitch which caused FontAwesome stencil disappear forever after few minutes using the application and being unable to reimport it. Evolus Pencil version worked well.