I used my te-wa breeze with 1 ounce overstuff to approximately 32 degrees and was VERY cold. In the low 40's to upper 30's i was doing alright with midweight poly pants and shirt (and a wool cap) but as it approached freezing i added a downjacket and i think a few more layers i had on and could not shake the chill. If there was no wind i might have fared better, but it was a fairly gusty night. The prior night was around 45 with decent wind and i slept well.
I love it in the 70's to low 40's but don't think i will try it lower without additional layering such as a uqp or thin pad below my back.