I did only first aid and questions for step I (a few years ago now, though) and did very well (if it matters, >250). Of course, you have to "do questions" appropriately - i.e. if you get one wrong, figure out why. Not only why the correct answer is the right one but why the incorrect ones are wrong. The wrong answers are usually there for a reason - to make you think about a certain syndrome or alternative situation.
For step II and III I did only questions and also did very well.
I think people can get screwed up by trying to memorize as much minutae as possible - because you can know all the minutae you can but still get a question wrong because you don't know why things are important.