How Well Do Performance Chips/tuners Work On Gasoline Engines

I’ve got a ’98 Wrangler with a 4.0L I6 engine and some of the guys at work were talking about performance chips and programmers today. They were saying that they work a lot better on diesel engines than gas engines but still might be worth looking into for my vehicle. Basically my driving style and needs vary a lot. Most days I’d like to save a little on gas and don’t necessarily need a lot of power while rolling around town, but I also spend a lot of time on the highway where I really need a little boost in power sometimes because Wranglers aren’t exactly aerodynamic and even with the 6 cylinder it can be very difficult to maintain 70 mph driving into the wind in 5th gear on 33″ tires. I know they aren’t cheap but how well do some of those programmers work, the ones where you can make adjustments on the fly without shutting the engine off?