A couple of years ago I was looking for a synthetic…
Temperature Rating: 20 F
Weight: 3.5 lbs.
Price Paid: $159
A couple of years ago I was looking for a synthetic fill bag to supplement my collection of down. After checking the products from Marmot, Sierra Designs, North Face, and Mountain Hardwear, I chose the Montbell Main Heat primarily because it had slightly more loft (which I still think is the main criteria for staying warm) than the others.
After extended use I believe this is a good bag and an excellent value (Backpacker magazine's comparison test agrees--if you can still find one. The patented elastaquilt stitching (also used by MH in their CrazyLegs series) does an excellent job of keeping you snug without the straitjacket feel. The zipper stiffner, hang loops, draft tube, etc. are all as good as anyone else's and the insulation has proven durable enough.
The temperature rating of 20 degrees seems accurate, and I have used it in the Montana winter while wearing a down jacket and fleece pants.
With synthetic bags, I really don't think there's that much difference between them in construction, so go with the ones that have the loft, insulation durablity, and compressibility.