I have used the original version Stealth spoken above in the 2008 comments. Also, upgraded and been using the latest version which is wider at the shoulders, the Sierra Stealth. I am 5' 11" tall and use the long size.
Both have kept me warm down to 5 °C with light layers on. Here in northern PA we can still get down to 2°C over night in early June. On those nights I might have to shake the down from the outer edges of the Stealth to the middle and puff it up there. Even then the Stealth has kept me plenty warm. A light thermal or fleece layer for me would definitely keep me warm with this quilt at 5 °C.
As with any quilt, the hammock allows for a nice seal on the edges when you tuck it in around you. That helps extend the lower temp range of this quilt. Being sold also to ground sleepers, 5°C is the absolute lowest I can get away with using it on the ground.
One other thing, the Stealth continues its use in the winter. I layer the Sierra Stealth inside a sleeping bag or another quilt to extend their lower temp limit ie -15°C. I dont carry a parka to wear around camp but layer the Stealth over my lighter jackets. If needed I put a larger wind jacket over it to break the wind. I have to go with multiple use on my items to get it all inside my pack.