I haven't tried outside in 0 degree weather yet but the bag is thicker than most cold weather bags I've seen. The stitching is off-laid which generally allows a bag to be warmer. It's constructed well.
Love it looks to be well made, but just got it today so havnt used. I can see why people may complain about zipper but dont see it as a real problem. I have 2 Slumberjack sleeping bags 30 years old that still are great!! (Although kids tend to have them more than I) Would recommend especially at this price!!
I slept in 30° weather with just a tshirt on and stayed warm. I'm 5' 11" and got the long bag even though the regular said it fits up to 6'. I'm glad I did, I didn't feel like I had much extra space with the "long" bag. It's a good deal IMO,I would recommend it.