So as a billy it's great. Lightweight, robust, swallows a large trangia perfectly. But sat atop a traditional trangia and it really sits too high to be of much use. It sits at the same height as a frying pan, but fill it with 4.5 litres of water, and you'll struggle to get it boiling in under half an hour, especially outdoors where all the hot air your burner is generating is being pushed out the sides by wind.
I use this when I know I'm catering for 3 or more people for boiling pasta or suchlike, but now take my trangia triangle as it sits nice and close to the burner on this, whilst using the traditional trangia to whip up a sauce or something. Done this way it's perfect, and all the gear still fits inside the billy when I'm done.
Definitely buy it, but carefully consider how you're going to use it. Keeping food warm, yes. Boiling 4 litres of water from cold on an ordinary trangia, think again.