Just to mention the switch from Nikon Fs to Leica Ms - There is a definite learning curve.
The focusing is reversed (clockwise versus counterclockwise) - sort of like switching from a Nikon to a Canon.
I find that framing and focusing is more of a challenge with the Leica (if you've been living in the Nikon F-world).
The other issue is metering. If you'll permit me an automotive metaphor - if you compare metering to transmissions - the Nikon F100 is a Toyota Prius and the Leica MP is a vintage Ferrari.