Love the app.
Would love to make it 5 stars + but if there was a timer after each puzzle comparing it to how fast u did it, if you did it before if you were faster or slower than the time before, your average speed etc...
Also comparing it to how fast other users do it.
Also different shapes on different puzzles would be cool, level of dificulty setting.
A great idea to increase interest would be to have the number crunchers figure your break even number, and then your desired proffit numbers, then look at your app sales histories, and figure out how frequently you could offer a $100, $1,000 and/or a $10,000 prize for best over all times by a user or what ever, you could have a crazy MMOC, massive multiplayer online competition,.
When doing your mid to long range discovery into the prize $, dont forget to look at potential sales of other apps through "strip advertising" at the bottom of the app, because of decreasing new sales of the app.
Sorry for the epistle, but I love your app, its very good at helping me focus on a potential solution when Im blocked on an idea. Im a currently unemployed executive whos needed to think through a lot of solutions and as I said before, your app has been both helpful and theraputic.
When you use these ideas, please think of me and maybe slip me an app or two...