The weakest game of the NES series, in my opinion.
First of all, let's talk about the robot masters,
In MM5, the robot masters seemed weak, and some of their weapons were horrible, like the Charge Kick or Napalm Bomb. Plus when you fight one, the boss music sounds ridiculous, not like it's bad or anything, just not what it should be.
And some of the names are just terrible.
For example:
Gravity Man
Gyro Man
Napalm Man
Crystal Man
And the stupidest name in this game: CHARGE MAN!
Seriously, what kind of name is Charge Man?!
Plus when you fight them at Wily's Castle, you know how you end up in another room similar to the one you fought them originally, well Capcom got lazy and put them in the same place as before. Although they say Protoman is behind it all, but anyone who's played the first four will likely think that Wily is just making Protoman look bad, and they're right, of course.
But their themes are enjoyable and great to listen to. The game has a great soundtrack, otherwise it would be a lot less enjoyable.
Now for other things:
Rush has been nerfed down horribly, especially the Rush Jet. And the Super Arrow is a mimic of Item-2 from MM2. But they added a new character named, Beat, a robot bird that helps you defeat enemies easier once you collect the letter in every level.
While this game has many flaws, it is enjoyable if you are able to look past them.