This game is pretty understandably dated. It's the first game of the series, and an early platformer in 1987, things were still being found out. It has a number of annoyances in control, enemy and level design, boss design, etc., but I do enjoy the game as a first entry for it's weirdness... with the rewind, without it, I'd probably say this game is like and 8/20.

Here is my favorite bosses from worst to best

6 - GutsMan
5 - Fire Man
4 - Elec Man
3 - Ice Man
2 - Bomb Man
1 - Cut Man