Waste of time...
I heard all the comments before hand, but I ended up with one anyway (was donated to me) and I gave it a go on my 306 Rallye. My main aim was to get more MPG as claimed, as it aint that great on mine.
Well, it improved my mpg by 1 or 2 if driven carefully, but generally no change, it made no difference to performance and just made an annoying noise, so its still on the car now, but screwed shut so it doesnt work.
Trust me, youd be better spending your money on other things...
Ive actually got one & tried it, so this is actual experience rather than hear say.