I have a 20* slick and a 0* slick bag from kifaru. I have yet to use the 0. My wife used it on her deer hunt a couple weeks ago.
My 20 got warm most nights.. and it was in between 20-30 every night and morning. If you get a 0* slick, you don't want a -20*, unless you are a super super cold sleeper...
Mine are longs (i'm 6'4"), only thing i'd say about them is i wish i would've got a wide instead of standard width cut. shoulder area is too narrow, but it's not uncomfortable, just too tight for my liking. width on feet and legs and body is good until you get to the shoulders, then its too narrow. that's my only gripe and complaint.
They are very true to rating and durable. I've used mine for about 25 nights this season now and it's kept me plenty warm and dry.