In reply to bengough22:
I've used one up to 6400m at C2 on G1. Though there are exceptions, usually the Piezo lighter won't work above 4000m (sometimes lower). Performance definitely diminishes as you go up, especially over 4000m. Even worse if you don't use the regular cup/pot but the larger one - they're rubbish.
Canister stoves are nice and simple at altitude but performance really suffers - on all models - so they're crap for melting lots of snow. I also used a little Kovea titanium stove with a aluminum pot at 6300m in Tibet a few years back, in very cold temps, around -20C, and that was worse than the Jeboil on G1. In each case boiling a litre would have been both impractical, unwise and impossible so I have no time I can quote.
D