I used to work at the Hollywood Video here in town, and I'm good friends with everyone who still works there so I usually get some perks. I was told that it's one free blu-ray rental PER VISIT. I'm sure it simply depends on the employee you get.
Also, in case any of you didn't know on your birthday you get one free rental. It can be a game, DVD, or blu-ray. You can get multiple free rentals if you make multiple trips because each time your account is brought up it says Happy birthday and adds a free rental. You just have to make sure the next time you go in it's a different shift or you rent from a different employee in case they recognize you.