If you mean the sleeping bags with armholes in them and a zipper down the front - they are fantastic. My son regularly kicks the covers off and moves all around the cot - at least with the sleeping bag he stays warm - as it's getting quite cool now.
I've got one, too! Otherwise I keep having to cover up ds at night as he kicks off his covers. If you meant a Nano bag, like you put in a car seat. I've got one of those too. Works GREAT! Don't have to use a jacket or a bunch of blankets. Just put him in the bag in the carseat.