The basic functionality is great, but having the ability to make clickable online demos where I could send links to people would be a killer feature that would make me switch very quickly.
Having said that, a few UX notes:
-You have incompletely applied the tab metaphor on the left-hand panel. For a while, I was trying to figure out why a label was a stencil but a paragraph was an image. At the very minimum, your "shaded" tab should extend under the other one to show that it applies to both columns.
-Categorizing your stencils, even a little bit, would help. Balsamiq screwed this up, and now they have a bunch of nonsensical categories (quick, what would you expect to find under media?)
-I would recommend grouping your stencils by things that tend to be used together. For example, form elements should be all together, mobile stuff too.
-I know you're not native, but the right-click context menu on Balsamiq is much better than putting the align/arrange/lock at the top of the screen. I use this stuff a lot, and the extra distance is a pain
-Also, I don't see the capability to have some sort of templates. That's definitely a must for any decent size project
/edit I just discovered the context menu (double delayed click is a good solution). Still think at least the lock button belongs on the menu and not at the top