I'm sorry to say, but I don't perceive it to be useful at all. I'm probably in your target demographic - college student in engineering, and I have to tell you that anyone who's going to be doing any sort of work with graphing calculators would prefer to buy one than use their iPhone. Here's why:

- Physical buttons: when you're chewing through problems you absolutely MUST have physical buttons with tactile feedback. Like the calculator app on the iPhone (the default one) the buttons are way too small to even begin operating quickly.

- Battery life. My exams are often 3-4 hours long, and an iPhone would not be allowed as a legal calculator. Even if it were, the iPhone would have trouble holding a charge through the entire exam.

I just don't see much of a market for it. At a lower price point maybe - I see this app as being useful for possibly one-off equations, but nobody who needs this sort of software will use this app as their primary graphing calculator.

Now, what I would gladly pay for is a graphing calculator software that comes with a USB keypad. I have my graphing calculator with me always, but sometimes it's slow and the screen sucks. It would be wonderful to have the friendly physical user interface of the calculator and the power of the PC.