I've got the 20 degree Draft Dodger which is a Rev X with the addition of a draft collar marketed by Seek Outside, and love it. I just got it a couple weeks ago, and have gone down to the mid 40's in it. If it's above 60 then you'll definitely need to vent as I had to vent in the high 40's with mine, and I'm a cold sleeper.
Fortunately it is very easy to vent. I threw one leg out, then threw both legs out and just had them on top of the quilt, and could get both arms out as well.
For nights where the lows are 70's or above I think I will sew up a rudimentary quilt from a silk bedsheet and call it good, but until that point I'll be in the Draft Dodger.