I have seen both, but only been with uncircumcised. In the US, it's unusual to see uncircumcised, whereas I hear, in the UK, it's the opposite.

The foreskin has a lot of nerves, so when it's cut off, those nerves are removed. Some say it would improve sex with women, because it would make men less quick with sex, since they'd need more stimulation. On the other hand, a lot of women would say, men don't need more nerves down there, LOL! (Yet, women actually have more nerves in our genitals.)

There are religious reasons for getting a male circumcised, but the health reasons have been found unnecessary. As long as a boy learns to keep his foreskin clean, he generally won't have problems.