My FM2n is my least used camera. My criticisms are mostly about the viewfinder:

a)The shutter speed display intrudes into the viewing area! I understand why Nikon did this; it's so the numbers are backlit. I just don't like it.
b)The shutter speed, aperture, and meter indicators are each on seperate sides of the viewfinder. I'd much rather have them all on one side.
c)The motordrive, while extremely comfortable in the hands, jams when misused. It jams on me all the time because I forget to keep the advance lever in the off position.

All that said, I have enormous respect for my FM2n because it appears to be indestructable. I've dropped mine several times, it's been kicked, left in the car for weeks at a time, left unprotected in the rain and snow. It doesn't give a f&$%. I've had to send my F2S for repair after it's been dropped but never my FM2n. It could just be luck, but I think most users have similar stories.