I bought 9 month size for my four month old baby as he's starting to outgrow his six month size clothes in other brands. After one wash, these onesies shrunk down to the six month size he's growing out of! I wish I would have seen the other review before purchasing, I would have bought a larger size.
It is really nice material, though - has some stretch to it and is thicker than the usual cotton onesies. I do like that.

So overall I think this is a quality item- just be sure to buy it a size or two bigger than you need!!