Worked up to 16K feet and 10 degrees
We took the Helios on a trip to Peru, 6 days on the Ausangate trek, all camps 14.5K and above and very cold. Water bottles froze in our tent. The stove worked wonderfully, all the way through a canister. I've had MSR XGK, Svea 123, Peak 1, etc. and even other Jetboil stoves. Although a bit heavy for two people (we had four), it is a joy to use.
This does not flame up like a white gas stove, but it does flame until warm. Ignitor worked fine, although it did break on our other Jetboil. Mostly we boiled water and it is awesome for that, but we also had to simmer soup for 30 minutes (we brought dried soup mix to give out to Quecha locals instead of candy, but we ran low on food and tossed that noble thought and ate the soup). We turned the canister over (right side up), and it simmered perfectly. Great stove.