Well, if you're talking about those, some are worth a little.
Two of those directly use pressure from in front of the radiator to feed the engine. Seems fine at speed but I suspect these might pull hot air off the radiator at low speeds though and actually reduce power.
The SLP is a true ram-air intake, but nobody's done a thorough test on it. I gained 2-3 mpg highway in cold weather, and my gas mileage is very temperature dependent now, so I know it works. How much? Dunno. Worth the price? Dunno.
None of them will ever show a gain on the dyno though, for obvious reasons.