rock n play

Hi Ladies!
I am having my third, but the last time we had a baby the rock n play sleepers weren't around. I have heard so many great things about them, and a lot of people recommend them. I am just curious if anybody has heard whether or not it is safe for overnight sleeping? I have always just heard that baby should be flat on Their back with nothing surrounding them to decrease risk of sids. 
I am not questioning anybody's choice to use or not use one for overnight sleeping- I'm just trying to get all the right info. :)