Why did it take so long for sleeping bag engineers to come up with this integrated bag/pad concept? This feature alone makes the purchase worthwhile. On a recent canyoneering trip in Moab, UT, I combined the Zirkel with the Air Core REM and my BD (Bibler) Hooped Bivy for one of my best nights out ever. I was actually a tad too warm in the Zirkel (unzipped the bivy & bag to vent) on a night that was clear and about 35 degrees F. The Air Core REM in the Zirkel sleeve was great--I couldn't feel the ground and woke up still on the pad (a rare occurance for me). This is the best valued bag I could find and have been completely satisfied with the bag and even more so with the system.