I've been thinking about one of these myself.
Truth is, I've done a bit of research, and personally, I wouldn't go for it.
Reasons being are:
a) They don't provide much boost (air pressure) thus you don;t see much of a gain
b) Why would you want some peice of plastic crap which could easily break off and enter your engine causing major costly damage?
c) They are also a disadvantage as when it is turned off, it disrupts and blocks the air flow into the engine actually causing less air intake.
So consider all these before spending over £140 on one