... So I gave it to my daughter and I kept my old MK250. The MK520 is not very silent keys as is noted, more buttons are quite "hard" and finally it often happened that keystrokes are not taken into account. The mouse is a total lack of precision, despite attempts speed settings, sensitivity, ... I was looking for a keyboard like that of my Portable PC, silent, fast, ... but other Apple keyboards I think I would have difficulty finding PC (Apple keyboards with a different arrangement of keys).
My daughter is happy, she recovered a nice keyboard and a wireless mouse (she does not care a little he seems accuracy).
I would not recommend this set Keyboard / Mouse, Logitech had become accustomed to even better cheap rates.