Been experimenting with the North Face Fission for 6 months now and I've been extremely pleased with it. Specs as follows:
size reg = 33.9 oz w/ golite stuff sack (my scale)
Pertex Quantum shell, Polarguard Delta fill
1/3 length zipper
uncluttered, functional drawcords
$199 retail -- permit me to squawk a bit, I got it 20% off : ) ooh, ahh...
Anyway, I've used it in conjunction w/ a BMW cocoon pullover, Vapr Bivy and Oware Tarp. Great system--with a normal array of fringe 3-season clothing, it takes me near to zero. I don't know of a synthetic bag that comes so close to the warmth:weight ratio of down. The upcoming synthetic Arc X maybe....
Be aware that my success w/ this bag in low temps is due in part to the fact that it fits me like a glove. I'm 5,9 160 lbs, and wearing a baselayer, possumdown vest and beanie, windshirt and synthetic pullover, the fit is ideal for me. The cut is slim, tho, so others may not be able to wear as much clothing inside. 59-60" interior shoulder girth according to TNF, I think. (?)
The 1/3 zipper makes the bag unsuitable for temps much above 60*F, but in the 5-50*F range it's great.
Hope this helps.