GHommes wrote a review of Marmot Trestles 15 Sleeping Bag: 15 Degree Synthetic on June 4, 2012
I've had my Trestles 15 for several years now, and sleep in it 20 to 30 nights per year. It's a durable bag, well built, but the 15 degree rating is certainly low. A 30 F rating is more accurate, unless you sleep in long johns, a fleece jacket and stocking cap.
Like many bags, the width/volume of the bag is too large, so you have to heat a lot of dead air space. To be more efficient the bag should have the insulation closer to the body.
Perhaps sleeping bag makers are adjusting their bag sizes to fit the increasing girth of their customers.
All-in-all, it's a great bag at a decent price for late spring through early fall conditions here in northern Minnesota.