Its hell on your fingers. I have thick callouses and the sides of my fingers around the nail are pretty torn up.  Its all a matter of personal preference though.  I just switched back to oval inserts and its significantly more comfortable.

Depends on:

Grip Pressure (very important factor)

If your span is "correctly" fit for you (Too long or too short with incorrect pitches can be a toughy)

Your timing and how you throw the ball (if you are late in timing and pull the ball through)

What you do at release (let it roll off your hand or "help it a bit"

How your skin reacts to the resins in the balls

If you leave them razor sharp off of the press (which I do) or buzz them a bit

Hope these help a bit!