A bit late reply. We too bought the dimple pillow to prevent flathead syndrome (overworried 1st time parents who wants to spend some money, haha).
However, my son doesn't like it and he doesn't stay "in" it. He would eventually move about a little and wiggle his way out of the pillow. Initially I will put his head back inside if I noticed he is out but sometimes it will accidentally trigger him to wake up. We don't enjoy the experience of a baby who got his sleep disturbed so we eventually stopped lifting his head back inside and moved on using a thin memory foam-like pillow. He likes that a little better but even so, he moves about when he is sleeping and started to enjoy side sleeping, then no more lor. The pillow is there in the middle but he rolls and side sleep at the sides (not on the pillow).

Now the back of his head is not the roundest but maybe him napping in the craddle/sarong helps to round the back of his head more.