My son (11) and I spent an hour or so outside during our "artic blast" that came through the area, testing our newly acquired Montbell UL SS #0 (zero degree F down) sleeping bags. The temperature was ~ 2 degrees F with wind gusts in the 20-30's. We both were wearing the following:
Feet: double smartwool hiker socks
Legs: thin baselayer, 200 wt fleece pant
Torso: thin baselayer, 260 wt merino top, micropuff pullovers
Head: mid weight beanies
Sleep pads: regular length Therm-a-rest Prolite 4 plus two layers of Gossamer Gear Evazote 3/8" thinlights
Overall, we were completely warm (I could have easily not worn the micropuff) and with exception of our noses, did not feel the wind gusts.
In conclusion, these bags rock! They are very warm, windproof, and for less than 3 pounds, not bad on weight. I think I could take these bags to at least -10 to -15F with the clothing system we had on.
Here we are bundled up: