I have a K&N Cold Air Intake. I would not be able to run 33's in overdrive if i didnt have it on, i got about 1-2mpgs better than stock intake, and a great sound under hard throttle! its very simple to install took me about an hour but i had alittle more todo since i have a TJ. but not only will you save money with gass but by not having to buy paper replacement filters ever again is great too!

 

I have the xdi tru flow. It performs as advertised. There is a seat of the pants improvement. Happy so far.

 

I put on an AEM last December, cause I visited the Summit Racing store in Atlanta and refused to go away empty-handed

Anyhow, here's what I noted:
1. Does it improve power? Noticed it on day 1 in the 3000-5500 range, but got used to it after day 2. Probably a 5-7HP gain at most.
2. Does it improve MPG. Got a whopping 1/2 mile improvement testing over 3 tanks of gas. Should pay for itself in 25 years at that rate
3. Does it change the exhaust note? Yep, acutally makes the 3.8 sound pretty darn good for a 6-banger.
4. Does it look/fit good? Yep. It looks darn nice under the lood and rivals the K&N FIPK II on my LS1 Camaro.
5. Is it worth $250? Not really. Aside from a better exhaust tone + a few ponies I've already becomed accustomed to..... and a pretty engine bay (though I'm not usually popping the hood on my Jeep).....

That should answer your q's