How bad is left-foot braking for the car? Not arguing about the effectiveness of such a technique but it seems to be devestating - huge stress on brakes, driveshafts, etc..

Yes, it would put a tremendous strain on the engine, BUT ONLY if you are braking and applying throttle together. Anybody, feel free to correct me:D

I Rally a Legacy with standard transmission (getting towards the end now) but i use left foot braking all the time and apart from brake wear i have never had any problems.

The only over problem that you can get is that on standard brakes the heat will worp the discs in no time

A black art. Little or no benefits in road driving. Added brake wear, transmission strain and fuel consumption being the obvious ones. Add the number of times you crunch the gearbox due to missing the clutch, or boil the brake fluid, or glow the discs, or lock all 4 wheels when you slam the brake pedal thinking its the clutch, and your nice shiny road car will not last long.

Do it well (and it takes a HELL of a lot of practice and tuition before the art is perfected to the point when you are using it to full effect) and you may get home several tenths quicker than normal.

Now if you are going rallying, thats a different story..

I use it on the road nearly all the time when I have Avis hire cars. Doesn't half help a 1.8L Vectra turn in on roundabouts :)