I have a 2001 F-150 4.2L 6-cyl. My dad bought it in 2002 with about 15,000 miles on it. He ran it for 12 years loaded with all of his work equipment, which was a pretty heavy load. When I got it about 2 months ago it had 285,000 miles on it. As of now it has almost 289,000 miles on it and I'm confident I can get it to 350,000 miles easy. Original engine and tranny. The only things that have been replaced is the rotors, oil pump, drive shaft, and basic maintenance. Last month I ran it into a guide rail and other than a broken headlight and a bent bumper it was fine. I rely on this truck to get me to work and college on a daily basis. Very sturdy and reliable.
This is the first new vehicle I have ever purchased. After selecting a white,F150XL 2dr LB model, I added on the 7700 payload package which increased my towing GVW to over 10000lbs. It included a 3.73 gear diff. I tow 2 horses regularly with no problems. I've even towed a 15,000lb Skid Loader (Although that was a workout). I've never had a single problem with this truck. The truck is plain durable. I figure about 12-15 MPG when towing a 5000lb horse trailer. I'd still purchase this truck all over again unless I could afford to go diesel.
This is by far the best vehicle I have ever owned. I have over 220,000 miles on it and it just keeps on going strong. Minimal maintenance costs. Well built truck. Replaced the clutch and slave cylinder, brake pads and tires. Thats it. Awesome truck.