We've only used it on one moderately warm night, and our baby was a bit warm.  Otherwise, it's been great.  You should heed the washing instructions...I washed in hot and it shrunk.  She won't be able to use it for 2 years.  She is so mobile in her sleep a sleep sack is the only practical way of keeping her covered while she sleeps.  I think it could only be better if washing instructions were located ON the sack and not just on the packaging materials.  My memory isn't that great. :)