As an engineer, I like crunching numbers. Since I got my JetBoil, I've been boiling different amounts of water and recording how many grams of fuel is required at different temperatures ambient/water/amounts/etc.
In my experiments I've noticed that there is little, very little extra heat. There is no wasted heat to warm your hands. Now that's efficient!
French Press works like a charm.
This is the smallest and most efficient stove I've ever seen.