Is Crossfit good for building muscle?

Cross fit is merely a mix of other disciplines mashed up together into a sport of it's own. On that note, you already have to be in shape to do it. As for building muscle, not so much because cross fit contains a lot of cardio and cardio and muscle-building do not mix. But, like I said, you're already supposed to be in shape if you ever get into cross fit.

Yes, but be prepared. It can be intense.