I used to be VERY picky about topics for potential sites and apps. I did a lot of research to make sure that my idea really is original. If something existed that even came close I gave up the idea and tried to find something completely new and unharvested, regardless how niche it was. In reality the probability that your basic idea is not absolutely original is extremely high and so is the potential for frustration. The good news is that the actual implementations of almost any idea are very likely to be ugly, practically useless or both. That still leaves you a great space to fill of creating a beautiful and useful app. Hurray.
If I have an idea, I do a very short initial research on the web and on the app store. If I cannot find something that is EXACTLY what I am looking for I take it as inspiration at most and try to identify its weak spots to avoid. Furthermore it’s always good to know your neighbourhood anyhow. Apps that you will be compared with, especially the big ones. That’s it.
If you find something similar, don’t get frustrated just find you’re own way and try not to mind too much. So I did a … list … making … app (!), maybe one of the most crowded, awful spaces in the app store and I just don’t care. The core features of my apps are managing list items on a 2 dimensional board instead of a linear listing plus a minimal appearance with a black background to allow usage in complete darkness without getting blinded. I couldn’t find both attributes combined on my non-thorough research, so I launched XCode.