Our 6-week old baby was born a good sleeper (thank heavens) but during naps and overnight started to kick off her blankets.  I was worried about this partially because it would usually wake her up when she got cold, and babies don't regulate their own body temperature well, so she'd be like a little iceblock, and partially because sometimes she would kick the blankets up near her face, which made me worry about SIDS.

Well, the Merino sleepsack is the best thing out there.  Yes, it's expensive, but it wears beautifully, the workmanship is awesome, it stays around her and is fine to transport her from crib/tired to carseat without disturbing her too much. She's sleeping much better, and so are we knowing she's at a lower risk for SIDS.

One thing I would point out - the best garment to wear underneath the sleepsack is a cotton sleep-n-play. We have a cold house, so we pull the built-in mittens over  Wearing synthetics defeats the purpose!

Edit: We had the sleepsack until our daughter was 12 months, then purchased the 2-4 years version.  She didn't get much use out of that one (she's an active sleeper these days) but these sleep sacks wear so well that our new baby is wearing her old one now.  We have two and air-dry them when we wash them on the wool cycle.  They show some wear, but for being worn every single night for a year, then every single night for the last 6 months (our little guy) they are really holding up as one of the most amazing baby garments of all time!