In my experience they're generally considered a gimmick.
They basically allow extra (unmetered) air to enter your inlet manifold.
The idea is that this extra air introduces turbulence therefore helping the fuel and air to mix more thoroughly and alledgedly this will result in more complete combustion.
Some people believe they have seen a slight increase in economy but I've yet to have heard of someone that has gained any performance advantage.
At the end of the day, it leans the mixture.
It's up to you whether you believe the turbulence theory, apparently is used in F1 but I would imagine that they integrate it more fully with the fueling system and therefore take into account the required mixture adjustment, rather than do a DIY bolt-on job.
The manufacturer's site doesn't really give you an independent view, perhaps you could try talking to a good (trustworthy) tuner and see what they say ?
