Hey, I just bought that same North Face 40 degree bag last week! But I got it for indoors because my 10 degree mummy bag was too warm for that use.
Having slept in it for a few nights in indoors temps in the low 60's, I can definitely say I will NOT take it on any outdoor camping trips north of Biloxi. It's pretty thin, it has no flap to keep cold air from coming in at the zipper, and the big rectangular foot area takes a long time to warm up my feet.