I was simply using it on the iPad, but generally I can't imagine the iPad is great for mock up design (unless you do some really cool multi-touch things).

Generally, the use case for mock ups at least where I work (we use Balsamiq) is that everyone at least has a laptop.

I'd work on getting parity with Balsamiq, and focus on doing other things better. We use Balsamiq at work, and generally we have a lot of assets already in it (I know you are working on importing), but you'll need to unentrench their current user base.

Going on the freemium model will disrupt a lot of their income too. Right now balsamiq requires that you pay to use it even if you are not collaborating, but I'm sure you already knew that.

I use my iPad for almost all my initial mockup work. There are some great apps out there (iMockups, Mocking Pad, perhaps others). I find I'm more creative when sitting on the couch working on UI design than I am hunched over a desk. The apps do a pretty good job enabling pixel adjustments in a touch environment.