I've used the Helium. It's pretty accurately rated, unlike the WM bags which are about 10 degrees conservative. I ended up replacing the Helium (and a Hydrogen) with Western Mountaineering bags because of the half zippers and the better warmth. I've used the WM bags (particularly the Mitylite) as a quilt when the temps were such that using them as a bag made them too warm.

Last week, I tried out an MSR Ventra quilt, rated to 50 degrees. The temperatures where we hiked were 45 and 50 degrees on successive nights. The rating was very accurate; I was comfortable under it in shorts, T-shirt and shorts on the warmer night, and on the colder night I stayed plenty warm in a set of lightweight longjohns. It may just become my new go-to bag, because I could "sprawl" under it.