I had a chip from Superchips in my '98 F-150. While it was a nice gain and firmed up the tranny, I think that the K&N Air filter and Cat back exhaust did just as much to improve performance.
On the diesel trucks it is a totally different story. A $400 tuner makes a huge difference - to the tune of 100+ HP and 200+ lb*ft of torque. Dropped my 1/4 mile ET from 16.0 flat to 14.33 and improved my mileage while towing by 10%.