We have spent quite some time selecting from this category of tools lately for a large project, so have some current experience with most of them
Balsamiq and iPlotz are getting all the attention lately and they are both very good tools for simple sketchy" look mockups, and while we found the sketchy/hard-drawn look a HUGE benefit (well written about elsewhere) we also wanted more interactive demo features.
We then found FlairBuilder and it does what the others do just as quickly (also an AIR app) except that it goes much further (if you want it to) and creates interactive demos that actually run ie. the controls are all real windows controls.
My main issue was that is doesnt have a "sketchy" look, and after a few pleasant exhanges with the sole developer he has agreed to add this as a theme which I believe is almost done.
Why having a sketchy/pencil theme its so valuable is causing reviewers to not focus on detail (colors/fonts/alignment etc) and more on function & concepts, as well as to stop manager thinking that its "almost done" :-)
See the history comment with the developer here
getsatisfaction.com/.../pencil_sketch_styles_as_used_by_balsamiq_and_iplotz
We own licences for iPlotz, Mockups, Axure and now FlairBuilder, and find we use FlairBuilder the most now.
But each to their own!