As I said, I hit Javadocs 3 to 4 times a day.
Jump to another class: it's in there. I don't use it because I find the dired mode more useful.
I've used several IDEs with syntax checkers. The utility compared to simple highlighting is not a large delta, compared to everything I'd lose if I stopped using emacs. I believe someone's written an incremental compile mode for emacs, too.