It is said that a cold air intake will offer increased performance, improved gas mileage, go-fast looks, and a great sound. The Tundra K&N cold air intake works by increasing flow into the engine while at the same time attempting to ensure the in-flow as cool as possible. 

For many years, K&N has offered a less expensive alternative to cold air intakes. K&N assert that their high-flow air intake systems significantly increase air flow and therefore offer many of the same benefits as more expensive full-fledged intake kits.

The good folks at TundraHeadquarter.com installed and tested the K&N performance air intake system and air filter in a 2007 Toyota Tundra double cab with the 5.7L V8.  According to their review - "the K&N air intake filter showed an average 0.6 mpg improvement. Extrapolated to 100k miles of driving, that works out to about $1100 in gas savings (assuming gas costs $4 per gallon)."  

Here's what TundraHeadquarter.com concluded from their K&N Air Intake Filter test:

• Performance. While there was no significant gain in measured horsepower from the filter (all our test runs were within normal hp variance), there was a noticeable improvement in throttle response and engine performance at cold start.
• Fuel Economy. Get your wallet ready - if you don't already have a K&N air intake filter (or an air intake system), buy one now. We noticed more than a 0.50 average improvement in mpg on our test vehicle over the course of 2 weeks and about 800 miles. Before we added the K&N air intake filter, our test truck averaged 14.3 mpg. After the air intake filter was added, the test vehicle averaged 14.9 mpg. That means that over the course of our test we saved 2.3 gallons of gas. Put another way - spending $50 on a K&N air intake filter that improves your mpg 0.60 will pay for itself in about 5,000 miles - if not sooner. If you extrapolate out to 100k miles, our test results show that the K&N performance air intake filter will save you about 280 gallons of gas. That's about $1,100 in gas savings.
• K&N Million Mile Warranty. If the gas mileage improvement isn't enough incentive for you to buy a K&N air intake filter, here's another. K&N's performance air intake filter comes with a 1,000,000 mile warranty.  As long as you clean the air intake filter as prescribed (every 25k-50k miles), it will last forever. The factory air intake filter (made from paper that is more restrictive than the cotton gauze K&N design) should be replaced every 15k-30k miles, at a cost of $20 per Filter. The cleaning process does require you to buy a $20 re-charger kit, but even if you pay $60 for the K&N air intake filter and $20 for the cleaning kit, you still break even after replacing four factory air intake filters (which can take 60k-120k miles, depending on how dirty your filter gets). Not to mention the gas savings.
• Factory Warranty Issues. There are a small number of rumors out there that adding a K&N air intake filter (or a similar filter) to your vehicle will ruin your mass air flow (MAF) sensor. Some of these rumors are true - if you clean and then grossly over-oil your K&N air intake filter, the excess oil will enter your air intake and could foul your sensor. As long as you're careful, don't over-oil your filter when you clean it.
• Alternatives. There are a few companies that sell a performance air intake filter similar to K&N. AFE, TRD, and FRAM might be the best known filter brands. Since we haven't tested any other performance cotton-gauze air intake filters, we can't say for sure that these other air filter brands offer the same benefits. We would definitely recommend the K&N air intake filter over the TRD unit simply because of the cost.  

End results, if you like fuel economy and better throttle response, buy a K&N performance air intake filter. They cost about $50 now, and between the million mile warranty and the gas savings (we calculated $1,100 dollars worth over the course of 100k miles), it's one of the biggest no-brainers in the history of the Earth.