1) compared to all other manual nikon bodies - it is practically new body, so chance to get bad sample is really small --> FM3a will serve you for 20-30 years at least (when FM, FM2, F, F2 were new same applied for those bodies)
2) Only camera on planet that has aperture priority and all manual shutter speeds when battery dies.
Of course it is expensive. In a way it is like Leica: great but expensive - but if money is not a problem, go for it .
ps. I don't have FM3a - I have F bodies - it is because FM3a does not have 100% viewfinder - that is a must for me.