Mario 64, only better.
The base levels in the game are massive and take quite a while to explore. This can be a bad thing if levels are poorly designed and repetitive. I was never bored to death playing this game. The costumes that you wear to gain different powers was a brilliant idea. I sometimes find myself using the wrong one intentionally to see if it will give me some kind of advantage.
The bosses can be quite difficult at first. But as you play for a while you realize that every one has a very specific pattern (not a new thing in video games, I know, but noteworthy as it ads a false sense of difficulty that I like) and if you figure out the pattern you can beat them very easily.
Overall one of my favorite games of all time.