I use Aperture. As for learning to use it, the curve was a little steep. (Before this I'd only used Photoshop, and not really to edit photos as much as embellish them). That said, I've gotten pretty good at it, through a combination of practice and reading online tutorials. Basically, I went in one step at a time, learned one trick, then another, adding on more as I grew comfortable with the things I'd already learned.
As for knowing when to stop? I still don't know the answer to that one. lol