I've had one of these fitted for 2 months and immediately took it off!! Avoid it with a barge pole!
You don't get any improvements on torque or acceleration at all and it doesn't actually cure any flat spots my car had. Also when you took your foot off the accelerator it made a really loud rasping noise which actually makes your car sound like it has a problem - not very sport like at all!!
After I took it off and had a look at the unit, I took it apart to find that actually the inside of the valve the spring had corroded and was very stiff and the actual valve head was clogged with carbon!! A big dissapointment and I won't recommend these to anybody. Fitting was easy enough, even though I had to sever a vacuum pipe to get it to fit, but overall it did nothing I could see that made it a better drive. I've also heard from various car engine experts that over long periods of time, it can actually mess up the internals of the manifold/ plenum chamber.
After I took it off and put it my engine back to standard again and got a K&N panel filter the feel of the engine was amazing again. I can now feel something when I put my foot down, instead of slowely creeping up to 60mph. No gay noise, no changes in fuel consumption and no flat spots!
The only thing I can say that is positive is when you get a colour coded one with the P-X breather filter on it, it does look like a decent engine mod that looks like it does something. But in reality I think it's a overstated product and should be revised on the drawing board!!