I bought a Summerlite earlier this year, and although the lowest temperature at which I've used it is about 40F, I could tell that if it dropped much lower, I'd be putting on (at least) long johns. The Summerlite is a fine bag for it's intended use, but it has some limitations.
It's a little underfilled...the down is very easily shifted to top or bottom with a gentle shake. This is a legitimate way to make a bag, of course, but the tradeoff, as always, is warmth for weight. I think my bag is typical, BTW, as it weighs just what the manufacturer states. Fill weight for the regular length Summerlite is 9 ounces...10 for the long length. It's rated at 32F, but I think that's optimistic unless you're a fairly warm sleeper, which I am not.