It depends what you mean by "good use", I suppose; I started out with just the very basic outliner and checklist features, and expanded from there. It didn't take much more than knowing those features existed, and what they looked like, to start using them. You do need to refer heavily to documentation when getting started with more complex features, but that seems like a fair tradeoff to me.

I'm also not sure what you mean by "complexity penalty vs. using a plain text file", since Org files are plain text files, with all the magic implemented in the UI layer - I regularly edit Org files on my phone with Editorial, and while the UI is obviously rudimentary by comparison, such editing is not actually difficult to do. Will you elaborate on what you mean here?