I had two version of Twiddler for a while: one with standard buttons and another with the buttons that I filed to make them concave. The one with concave buttons felt a lot easier on the tips of the fingers. This is probably because the surface area of the contact between a finger and a button is several times larger. Thus the pressure on the finger tip is a lot smaller.
Recently I filed buttons on the second Twiddler too and I feel a rather significant improvement how my finger tips at the end of the day.
This is how the buttons look: