Anyone actually buy one of these? I see them on Ebay all the time, promising 10+ HP, and better fuel management to boot. They run about $50 -- $100, depending on which manufacturer, and what type of air filter it comes with. Do these things work?

what engine do you have?

I run an intake tube or open style air cleaner on all my cars... they feel like they add something and if you stay easy on the pedal can actually help your MPG a little, but if you're heavy on the pedal, can make your MPG worse. At minimum you'll get a much better sound if you like a little more noise.

I've got the 4.7, and admittedly I'm a little heavy on the pedal. Sound and a little more HP are always nice, but my 04 Dakota came with AWD ( with 4 high and low ) so the MPG are already pretty bad, about 13.3 on average. I think I'll pass on the intake.