Hey Rich,
For a tenner, the flask is O.K.
It would do fine in sunnier, drier climes. But here it doesn't fair so well.
It is a lot smaller then a kelly kettle, and better suited to backpacking use. You have the ability to boil enough water in the bottle for 2, and cook in the cup. Ideal for my fishing shoulder bag, and if it worked well, would be the best thing ever. Unfortuneatly, that isn't the case.
I'll compare it to my 2-1/2 pint Kelly kettle. By comparison; it is a huge hassle to use. It takes about 10 minutes to get the boil, and you need oven gloves to pick the bottle up!
The base unit will also topple over with the presence of any wind, or careless hands. It's totally unstable when you take the bottle / cup off.
The main problem is that it needs constant attention. It doesn't pull through air, and thus does not burn hot. You can stick some longer, thicker twigs in the sides to raise the bottle or cup, and create a larger exhaust. But it's a flawed design.
In practise, almost any alternative does it better. If you want to burn natural material, make you own (drill some holes in a trangia mess kit) or use a Kelly kettle. An esbit and a metal cup still rules the roost in my backpack when I can't have an open fire. The Kelly kettles for the car.