In my twenties I've caught a fungus down there. It caused some scarring on the foreskin. Basically it got tighter, so much so that undoing (what's the term??) it during erections was painful.

The doctor said I needed to be circumcised, so I got the surgery. Healing was not too bad and I can't say it had any lasting impact (in sensitivity or otherwise).

EDIT:
I lie. There is a lasting impact — I can no longer wank without lubrication. With foreskin I had no need for it.