I have an idea for an iPad app...constructive criticism needed

I have an idea for an iPad app and I am about to let a professional developer implement the idea.

But I am unsure if my idea is worth the amount of money I am going to spend on it. The developer thinks it's a great idea - but he's obviously likely to be biased.

Is there any professional developer out there who would be willing to provide some feedback on the idea? - Is my idea worth fighting for? Do you think it's an app that you would use yourself? - How much would you expect the implementation to cost?

Any help and constructive criticism would be much appreciated. Just let me know in the comments and I'll contact you if you're interested.

Cheers -sabertoothed (from Hamburg, Germany)