You will either love or hate the Big Agnes "sleep system." You might love it if you're a back sleeper. As a side sleeper it didn't work for me.
The sleep system means that there is no insulation on the bottom of the sleeping bag. Instead there's a pocket that holds your mattress. There are several problems with this. When you turn, the sleeping bag doesn't move with you because it's attached to the pad. When you turn over inside the bag, your face is now buried in the hood. You can't draw the bag in close to your body, again because it's held rigidly by the mattress. This creates large air spaces around your body that you can't close up. The foot in particular is a huge dead air space.
On top of these problems inherent to the design, backpackers report that the Lost Ranger 15, and Big Agnes sleeping bags in general, are overrated for temperature. They say that the Lost Ranger is comfortable down to about 30 or 35 degrees. For that temperature rating it's quite heavy.
There are better sleeping bags. Consider the Lost Ranger if you're a back sleeper but be careful of the temperature rating.
