I used the Summerlite for 2 weeks in Patagonia in Feb/March. I was a little skeptical when I got it because, well there's just not much there. It does loft up to the 4" spec. Temperatures were often down into the 30's and while I was sleeping in a double wall tent, I never resorted to sleeping in more than pants and a merino wool shirt.
It's a snug fit, similar to the other narrow WM bags. Whether you like a snug fit and whether the bag fits you would be the primary considerations I think. I love mine.