There are a few military folks on here. As far as if it helps or not, the jury is still out. If a person just out of undergrad and a person with military exp were both gunning for the last spot in an available class, and everything else was equal, I feel pretty confident that the veteran would get the slot.
A couple of years in the military won't make up for a 2.1 UGPA and a DUI but I'm sure it is looked favorably on by the adcom.
I don't know if Officer counts for more than a private, but I would think it would. Based on the amount of responsibility each one has.