Marco Arment shares his thoughts on whether it makes sense for iOS developers to target well-established, popular markets in the App Store:

Whether you should enter a crowded market is complex, and it deserves a much more nuanced answer than simply, “No.” Yes, there are a lot of to-do apps. But Justin’s representative for this category, Clear, was released just a year and a half ago, and there were a lot of to-do apps then, too.