The new power ups (Jet which lets you fly for a brief while and Power which lets you sometimes punch back some shots to enemies) are great. That's the main gimmick of the game and it's really well done. However, I hardly had any reason to not use the jet all the time.
The bosses designs were some of my favorites, and the fights were nice as well.
The game kinda leans on the easy side but that's not bad, I enjoyed it throughly!
So far MM4, 5 and 6 are the best of the initial 6 for me.