Bought this when baby was transitioning from the swaddle to sleep sack (around 3 months). I got the fleece version for the winter, but baby gets so warm when zipped up and swaddled that I've had to lower the thermostat to make it cooler at night (not a bad thing) and he's dressed in a thin onesie underneath. Sizing is good. I like that his arms can be free when falling asleep, then swaddled after being put down. I don't like that the swaddle "wings" use Velcro. It's too loud when you have to make adjustments or remove them while baby is asleep.