The EM was a late-1970's product targeted for the entry-level shapshooter market - the folks who wanted more than 35mm point-and-shoot rangefinder and who were willing to deal with 35mm film cassettes, but who weren't prepared to pay the big bucks for serious SLR equipment. It was accompanied by the inexpensive series E line of Nikkor lenses - Nikon glass in plastic bodies and by a special 2 fps motor drive accessory, the MD-E. It had aperture-priority exposure automation with an electronically-controlled shutter, with one mechanical speed (1/90 second), but did not have interchangeable viewing screens. It was slightly smaller, and ligher in weight, than the other Nikons of the time.
I have a 100mm series E lens that I picked up at a flea market for $50 - and that is about the best $50 investment I have ever made. It's sharp, fast, and quite compact - a great portrait lens, and reasonably decent macro when coupled with either extension tubes or a bellows.
My sense is that the EM body was not as rugged as other Nikons of the time. That could be a problem today since I suspect that parts are no longer available. I don't know for sure what battery it used, but both the FM-2 and FE-2 of the same period used either alkalines or lithium batteries - so I suspect that you won't encounter the dreaded mercury battery problem that is common with older cameras.