The more doctors review the process of circumcision the more they conclude that it helps to prevent health problems from fungal infection to STDs.
Circumcised men who claim that sex feels better uncut are without exception unable to speak from any kind of informed position. From what I've heard speaking to men who've had an adult circumcision, the experience of sex or masturbation is generally believed to be more intense with a trimmed foreskin. However their experience could be just as much psychological because of personal issues they had with an uncircumcised penis.
The pain is the same for an infant as it is for an adult but a child won't be able to remember it and will heal much more quickly and much more reliably than a teen or an adult. Adults also face an enhanced risk of infection due to the changes to bodily proportion and hygiene after puberty.