I've now played all of the levels except Gemini. I've beat one (Top Man), the others I've played through until my lives were spent, which usually got me pretty far or to the boss. After I attempt Gemini, I'm gonna go back through them all the same way. At this point, I care more about getting a feel for the game than actually beating it.
The music is fun. I don't always notice/care about music in general, but some things just catch my ear (Mumford and Sons!) and I find myself just sitting at the MM3 title screen waiting for the melody change. The levels are also catchy.
Level design is interesting. I also don't like switching between my weapon and rush. However, the spinning tops platforms, the parachuting heads, cloud camouflaged missiles, etc. are fun and unique.
Gameplay... meh. It's rewarding to make it through a series of perfectly timed jumps. The jump/blaster moves are tight. I HATE the slide. It's probably just me but I can't get the hang of it and instead of sliding under the enemy I hit him. The slowdown when several enemies hit the screen is also killing me. Literally. The screen slows down and I can't aim at the stupid cat's bug children! The crazy thing, though, is that the whole is greater than the sum of the parts. Some snippets of a level are downright boring. Others might lag or otherwise cheat your way to a quick death. But the whole level, played through multiple times until you feel the rhythm (a.k.a. memorize it), adds up to a rewarding experience. But, I STILL hate the slide.