I actually have the 20 degree Zirkel mummy bag, and my previous comments are for the Zirkel. I used to have the 15 degree Lost Ranger, and I have found the Zirkel is actually warmer for me than the LR was.
I use a +30 Eureka Silver City for mid September elk hunting, $65 on sale. I have yet to get cold and have used it into the mid 20's. I also have a Cabelas XPG 0 degree bag that I have used one night. Woke up wet with sweat about halfway through the night with just base layers on. Stripped to shorts and had to leave it unzipped.
My main bag is not high end nor has a low rating but has worked well for me. I also use a short sleeping pad with my setup.
So I decided to buy the Marmot bag. Just couldn't pass up such a good bag for $260 on the military program. Plus I think in the long run the down will last longer