Rock n Play Sleeper Question

Anyone out there using the Rock n'Play for their baby? If so, how long did your babe sleep in it? Are you worried about flat head? Mine is 6 weeks and loves sleeping in it all the time. He sleeps in it at night and for some naps throughout the day. We put a bit of cushioning in it but I'm still worried about flat head. I'm also worried about transitioning to a crib since the sleeper is on an incline and wondering if he will get used to sleeping that way. Any feedback would be much appreciated!