I was looking at a K&N air filter (http://www.knfilters.com/filtercharger.htm) which they claim to be:

High Air Flow with Excellent Filtration
Designed to Increase Horsepower and Acceleration
Washable and Reusable
Will NOT Void Vehicle Warranty
Lasts up to 50,000 miles before cleaning is required depending on driving conditions
K&N Million Mile Limited Warranty®
Emissions Legal in all 50 US States.
Economical, a K&N Air Filter Will Last the Life of Your Automobile, Motorcycle, or ATV
Works with Original Equipment Manufacturer Vehicle Electronics
Easy High Performance Add-on
Environmentally Friendly

Has any of you used these? Do they really increase horsepower?

I have the drop in one that replaces the factory one. I doubt it increases horsepower, but filtration wise, Wet filtration is almost always better than dry filtration. Only reason I state this is because of the electrostatic properties of the wet filtration media compared to dry. I read a pretty good article in one of the science mags about this, and assumed because the KN filter uses an oil media it works on the same principle.
The other reason I have one is my brother suggested it and he's the mechanic in the family.

I’ve only seen 1 case of it on a J32A2. I don’t think it would merit an entire dyno session though, that’s probably why we have not seen much hard data. Another way to do it would be to see if MAF gm/s showed an increase with ONLY the filter being the change.

I remember that article.

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That's actually a great article. For all of you that have CAI's are you still running stock filters? Has anyone changed out to the Apexi or K&N's? That Apexi look excellent. It's not cheap either, looks to be around 70 to 80$ and I typically find you get what you pay for. Anyone have experience with these?

Will the Apexi fit on to the CAI that we already have? Some sites say that it only works on APEXI systems. They have a universal filter but there may be an adapter to retrofit to our CAIs.

That's actually a good question. My Injen says it has a 3.5" filter which is about 90mm. I only see 70 and 80mm options for the Apexi. But yeah an Adaptor might be a good option for using one. The Apexi's certainly look impressive.

i have a k&n i got for 55 if anyone wants to buy it they can... its not a K&N filter for a cai its for the stock unit..

it makes the car have a nice deep growl LOL

There are some gain with the drop-in, but not enough to notice by butt dyno. If someone wants to put a stock Max on dyno with a K&N drop-in...

FYI. You can get the panel K&N for our cars for around $35 on ebay.