It's really a personal preference - and your baby has veto power!
I used one made of memory foam with adjustable bolsters on either side (I got it from One Step Ahead catalog), and it was great for us. As my son got bigger, he started to slide down it, and we stopped using it at that point. Once we stopped, he was rolling all over the place and would get his arm/leg caught in his rails and start crying for a rescue. He was never hurt, just didn't know how to pull back out.
I saved the positioner for when he gets a cold, and I place it (without the bolsters) under his mattress like a wedge to elevate his head a little for easier breathing. I don't know how he feels about that, but when I'm sick I like to be elevated like that.
Follow your instincts: I chose to use it because I felt my baby was safer and more secure with it. If you're uncertain, don't use it. A baby will sleep anywhere.