I was looking for a bag for an early summer trip round the Tour of Mont Blanc and decided to give Thermarest's Auriga Blanket and sleeping system a try. I've always steered clear of backless sleep systems because I tend to toss and turn quite a bit at night and it always seemed to me that I'd be expelling air out the sides every time I moved.
The Auriga Blanket has a boxed foot that you can slip over the end of your mat to keep it in place then the open sides flare out to provide you with a duvet like experience. On warm nights you can sleep under it loose and when it gets cool you can resort to either one of two methods to keep it in place and stop the drafts.
I was using it with a Regular Xtherm Mummy shaped sleeping mat and purchased the Thermarest fitted sheet so that I didn't stick to it, and because it has loops sewn at the sides so that you can attach the sleeping blanket directly to the sheet. Thermarest give it a comfort rating of between 2 and 7C, but I think this is optimistic. The blanket doesn't cover your head like a mummy sleeping bag does, so at 2C I think you would have a cold head and need a balaclava or fleece hat just to stay comfortable.
It used to be the case that backless bags were designed to save weight, but with the Auriga, especially by the time you add in the weight of the sheet, its not THAT light a system. Its intended benefit though is comfort and its here that this type of system will appeal.
I used the blanket initially without fastening it at the sides. Temperatures on the TMB were quite cool, I don't think it got to 0C but it got close, and I felt cold in the bag and donned my leggings and baselayer (I sleep cold anyway and normally buy a bag that's rated about 5 degrees colder that I expect on the trail).
The next night I tried securing the sides of the bag to the sheet loops - these seem to be set towards the top of the mat/sheet combo which allows the blanket plenty of scope to be loose, but kept it broadly in place. In this configuration I found that as I turned over the blanket was lifted clear of the mat letting warm air out the sides - not great.
The next night I tied the blanket below the bag using cord - see photo - this kept the bag from lifting too far from the mat opening a gap for the air to escape. It worked - and I was happy and warm. However I would point out that in this configuration you almost have the same restrictions as a sleeping bag!
So, overall - what do I think? I reckon its a good option for summer use, especially if you tend to lie still during the night, you can get a real sensation of sleeping under a duvet rather than being cooped up in a sleeping bag. If it turns cold you can make the system more efficient using the fixing options, its not as comfortable, but if the weather improves you can revert back to duvet mode. Thus the Auriga Blanket offers real flexibility and comfort when camping during the summer and overlapping into Autumn/Spring when its not too cold.