The "swirling" effect that they tout occurs too soon in the combustion cycle to do any good. It is pointless to "swirl the air" at the airbox.
For any possible (but negligible gain) in performance, the swirling must occur at the intake chambers, where the fuel and air mix.
Some performance-oriented folks spend $$ to have their intakes "swirl-ported" to enhance air flow...but as Larry stated, this enhances carb'ed engines only where the fuel and air mix occurs at the intake runners.
Don't bother getting one...if you do, stick it in your lawn mower chute for more efficient swirl mulching!
