Right now, I'm the only one in our campaign that has a cohort, so the other players might use them differently, but I use the cohort to provide backup. I'm a fighter, so I told my DM that I wanted someone that could buff and use cure wands, so he gave me a cleric.
Right now, we're fighting in an arena, so my cleric patches us up afterwards, and maybe adds a few spells before hand, but doesn't actually enter the arena. If I was playing a mage, I'd take a fighter cohort, and use him as a bodyguard. Just depends on what I need.