So, we were discussing what are the factors of iPhone app success. There's so many apps, more than 50,000, most of them are POC, of course. It seems that what's important is to have users who certainly need the functionality, ready to spend $$ if they need the gadget and convenience of having an app always with you.
In case of a guitar tuner, we have musicians who may not be the wealthiest people around, but they spend thousands $$$ on what they need to fulfill their passion to music, such as guitars. I would never buy a guitar for $3,000, yet, that's exactly what my guitar instructor did. I doubt that he makes more money than me. They buy a lot of other things like strings, distortion pedals etc. A tuner is going to be just one of those gadgets they need all the time. If it's in iPhone then it's always in the pocket. Its functionality is far beyond any tuner you can buy, and it doesn't take any space at all! What also helps is that artsy people tend to like Apple's products.
I think that when developing a new iPhone application one has to think of these three factors.