The FM2 is possibly the best little Nikon body that Nikon has ever made. It does have some shortcomings.
The FM2 doesn’t have a viewfinder with a 100% coverage of the frame, it has about 92 or 93%. This is all right most times but it can be a bit dodgy when you use some interesting lenses, especially very wide angle ones.
The non removable prism of the FM2, whilst extremely strong, does restrict the cameras ability to work with microscopes comfortably, sports viewfinders so you can view the whole image in focus whilst wearing a motorcycle crash helmet, waist level finder for when the camera is hard up against a wall, or attach the waist level 6x magnification finder for super critical focus whilst using a set of bellows or extension rings.
Whilst the FM2 can do a mirror lockup, it can only do this by using the self timer, this isn’t a bad thing, but if you wish to use extreme wide angle lenses or for some other scientific applications, you do the need the mirror lock up facility of the F3. Mirror lockup is also very handy when using bellows and very long exposures.
The FM2 does have a motor drive capability, but it realistically only winds the film on, however the ergonomics of the camera with the MD-12 drive fitted, are really good for some things. The F3 with it’s MD4 drive, also winds the film on, plus it re-winds the film, if you desire, it can also be set to expose a continuous set of frames, up to a pre-set amount or number. The MD4 drive can also be set to a couple of speeds, plus you can also run the motor drive (and the whole camera) from a mains supply and also use a regulated frame speed with that configuration.
This mains supply system is quite useful when using bulk film backs, they do drain power. The F3 only has the possibility of a 250 frame bulk back, the F2 has the possibility of adding a 750 frame back, this holds a full 30m of film and it will drain a set of batteries from the motor drive after two lots of film goes through.
The FM2 uses centre weighted metering, most people will tell you that the F3 uses the same set-up, not really. The F3 has a 12mm centre ring on most of the 20 interchangeable focusing screens you can use. About 80% of the light meter light, is gathered through this very central section, effectively giving you a near spot meter capability which will vary between the different lenses that are attached, but generally speaking will give you somewhere between a 20° to about a 5° meter reading, very, very handy.
The FM2 does have a databack, but it is a basic one compared to the F3 which can have an array of choices, including the MF-18 which imprints the data in-between the film frames, instead of in the image area.
The FM2 has a nice small body, compared to the F3 and does fit into a pocket (without a lens) easier than the F3.
Extremes of weather, especially cold, are the bane of most things electrical with cameras, the F3 is no exception! The FM2 is the king when it comes to cold, it just keeps on going. In extreme cold I have used an external battery pack for my F3 to keep it going very well in temperatures around –25°C or lower.
I myself run a couple of F3 bodies, I suppose it is a piece of jewellery, perhaps I’ll ask my wife the next time she uses her F3 as to why she is using a piece of male jewellery.
PS:- the FM2 body is possibly the second best Nikon body for absolute functional ability, but it is certainly the best body for something that holds film and will keep on working day in and day out. The later FM2 bodies with the F-801 shutter (since 1989) are the best ones to get.