I've made most of my money on fitness apps, with a few utilities sprinkled in early on. I've been doing it full time for a few years, with freelance supplementing the income. I've written up yearly reviews on my blog[1] a few times. I've spent a little bit on graphic design, and I've partnered and done a revenue split for some of the graphic design.
That being said, it's far from being all roses. My sales have been down some the last few years despite continuing to update (some of the apps, not all) and making new apps. There is a ton of competition and there are definitely things I'd wish Apple would change.
Mostly though you have to think of it as any other business. It's not Apple's job to sell my apps any more than it's Wal-mart or Lowe's job to sell your products. Being in the store is the only thing any of them promise, and that is a huge boost for lots of products. If you get featured (on the app store, or maybe end cap at walmart) it's gravy. I have to work to make a great product, I have to get the word out as much as possible, and I have to do a good job of ASO (or SEO if you prefer).
