One summer I was stuck in Washington DC with my family on vacation.  I was 13 years old.  We had seen the primary sites, the secondary sites, the tertiary sites, and we were down to stuff like the NRA museum and the federal reserve branch that distributed pennies.  Or something.  Anyway, I turned to more important things, like shopping.  I saw a Nikon FG in the window of a store and thought, "that's the camera for me."  I never bought that camera (not like I had $150 or whatever to spend).  It was only this year that I acquired an FG (for next to nothing).  It is actually a pretty good camera.

Ergonomics/Controls. The FG is about half the size of an F5 and about a third of the weight.  This is not insignificant; the FG with a compact AI-s lens will fit in a coat pocket pretty handily.  The flip side is that it is tiny and hard to grip.  The FG's controls are well-placed, with ISO and exposure compensation settings around the rewind knob and a shutter speed dial (B, M90, 1-1/1000, locking A and P settings) coaxial with the wind lever.  There is a small switch that when turned on makes the camera beep when an exposure is out of range. 

I have two gripes about the ergonomics (other than size).  One is that the shutter speed wheel overhangs the front of the camera, allowing you to knock it off 1/60 (the synch speed).  The other is that the winding lever folds (presumably to be more compact), and you need a double-jointed thumb (I guess that's what the auto-winder contacts on the bottom are for).

Note that you have to turn the camera to M90 to completely turn the camera off.  Otherwise, light pressure on the shutter release will keep the camera switched on.

Finder. The FG's finder is actually pretty good (especially compared to today's mirror-prism SLR cheapo specials).  It has a row of LEDs down the right side that show exposure.  The focusing screen is a K2 screen (split-prism surrounded by microprisms).  It is pretty bright with lenses up to f/4 maximum aperture.  After that, the split screen blacks out.  There is an indicator for TTL flash.  The finder has an 0.84x magnification and shows 92% of the frame (what you see in a minilab print).

Metering.  The FG has very basic metering: centerweighted.  The FG augments basic manual, aperture-priority and program exposure with a backlight compensation button and exposure compensation dial.  The FG does not have an exposure lock activated by the shutter release.  It has TTL flash.  But the really cool thing about the FG is that it can do programmed autoexposure with AI-s and AI lenses.

WIth AI-s lenses, it learns the maximum aperture and stopdown rate from mechanical connections on the lens.  If the lens is not set at f/11 or smaller, two lights blink in the viewfinder (the equivalent of the FEE error in later Nikons).  It appears that the FG determines that a lens is an AI-s (rather than AI) by the presence of the absolute aperture lug on the lens.

With AI lenses, it's a lot more fun.  The camera actually does a best guess stopping down (remember, it only knows how many stops off of wide-open, not the actual aperture) and remeters with the lens stopped down.  This is exactly the same closed-loop system as the FA.  When you use AI lenses in program mode, the aperture you set on the lens is the smallest aperture the camera can use.  So if the lens is on f/8, the camera can only stop down to F/8.  Although I haven't fully explored this, it may be a way to use programmed mode and keep lenses from stopping down too much.  Ideal for keeping things in the "zone" for a particular lens and for controlling DOF.  I can tell you firsthand that this system works very, very well with AI-modified lenses. 

Bottom-line. The last question is always why.  For under $100, you can buy one of the most versatile (and accurately-metering) Nikon bodies for the glovebox, a backpack or wherever you want to stash a small, reliable body.  I think that the FG is a superior choice to the FM-10 (more expensive, poorly constructed, no TTL flash) and the FM3a (a lot more expensive, with little more to offer than a whole range of mechanical shutter speeds).  Just don't confuse the FG with the FG-20, which has no TTL capability and no programmed mode.