I liked it with the exception it doesn't have assets built in, you will need another tool to build the screens and then upload. Doesn't powerpoint, slideshare or similar tools already do this? On the other hand the comments feature and Share capability is a plus which the others don't have. Balsamicq is actually lanching their web based mockup tool which haves the ability to share, comments, import assets to use (edit these), similar pricing structure.