We used it when my DD was 5 months old and we were transitioning her from the RNP to the crib. It worked perfectly for us! From day one, she started sleeping 11-12 hours straight. She STTN in the RNP but when we tried to transition to the crib, it was two nights of HELL!
Anyway, we used it for about 6 weeks, and stopped using it when she was just about to roll over with it on. Tranisitioning her out of it was fine at first, but she started teething soon after and that began two months of not STTN. But I truly don't believe it was related to not having the sleepsuit on. She's now back to sleeping 10-11 hours through the night.
I would HIGHLY recommend the suit. It helped us out soooo much, and got her used to the crib. I used it for naps and night time, like I said it was great and well worth the money!