They are a high quality product and they might possibly give you a slight mpg boost but unless you're planning on keeping the car for a long time you're unlikely to recover your investment.

If your car is a 96 or later and you don't have an electronic MPG readout I'd recommend a ScanGauge for getting the best mileage.

http://www.scangauge.com/

Once you can see when and where you are using the most gas it becomes a game to reduce your gas consumption to the minimum possible.

If you already have a MPG gauge then use it to fine tune your driving style, there's more efficiency gain for less money in driving style than any other single thing you can do to a car.

Seriously, my daughter had an Expedition with the 5.4 and a towing package, she had a hard time breaking 11 mpg with that truck where I could get over 18 mpg over the same trip just by adjusting my driving style to use minimum gas.

But it was just one of a group of mods to it, so I couldn't speak to the effect on fuel economy. Mine installed with hand tools and it should be a DIY project. As far as it being cost effective, you would probably be better served by finding an enthusiast forum for your particular vehicle and asking there.

I use mine to help maintain a 30MPG average on my daily commute in my Focus.
As for the K&N, the difference between it and paper filters is the K&N impedes airflow less than paper. The reason for that is the K&N material is more open than paper. The oil you apply to the K&N is what traps dirt, only it does not do as good a job of capturing fine particulates as a paper filter. So with a K&N filter you will be pulling in more contaminates. For this reason I have not converted to a K&N on my daily drivers.

For me, the Scangauge taught me a different way of driving, especially how to use my momentum to best effect and averaging my speed. 27-28mpg went to 29-30 and then to 30-31 with the addition of Continental Pro-Contact tires (with Ecoplus technology!) That Ecoplus stuff is some good shit. OK, my post is turning out to be just a copy of Fumesucker's.

Purely city driving still kills my average. Nothing can be done about that cuz you're always having to accelerate from a standing start.

What kind of car do you have?

Hey, here is an MPG-only version of the ScanGauge. http://www.scangauge.com/products/scangaugee/

When you change the oil just take it to the sink and wash it,then clean the sink with BBQ Grille Degreaser.
I bought it from my former boss who rotates trucks every ten years,he got it wholesale from a dealer as it was a one year lease return.
The lessee apparently screwed up the OD to avoid mileage charges.
I would guess 12,000 miles per year after the first 100,000 mile year would be 340,000 miles.
It burns no oil and still has good compression and still smokes the tires into third gear which leads me to believe that the K&N Filter is doing it's job.
It was my daily driver up until a year ago when i bought a slightly newer Sonoma for better mileage.

They will save you money,period.
However,there is another brand which is cheaper in case you think you might sell later.
Dial up warning:[link:http://www.spectreperformance.com/