This bag is very comfortable but after using it in 20-30 degree nights I had to use a liner. It was not adequate for those temps. It was comfortably warm when it was 40+ but I would definitely get another bag if I was going to be in those temps again. The zipper that zips up the bag is horrible too and would always snag.
As far as the "Quantum expander", its not a huge difference really. I didn't even notice a difference when I zipped it up actually. I do love the length of the long though. It was perfect for me, as I am 6'3", 240.
The bag is comfortable, but I am not happy with it.