I just received my Kifaru -20 slick bag . I was surprised how much room it actually had. I am5'8" 240 lb and I almost ordered the wide bag because I like to move around at night. Glad I didn't because I have enough room in the normal width to roll over without getting bunched up. The foot box is fairly large as well. I could easily sleep with my boots on in it.
I have not had a chance to use it yet, but in about 40 degree weather it was too hot to keep zipped up after a few minutes. That brings me to the next thing I liked, the zipper. There are a total of 3 zippers on the center line so that you can adjust how much ventilation you want. I like the fact that I could pull my legs out while still keeping my upper body covered if my feet get hot.
I've used a few different types of synthetic bags, and if this one holds to the temp ratings, I think it will be the best. I remember reading that the first run of bags were cold, so Kifaru added more fill to correct them. Maybe someone else could chime in how well they handle rated temps.