I'll be the devil's advocate. They're not all bad or they wouldn't sell at all. OK, some are, but not if you buy from somewhere reputible and not a local fly-by-night internet garage. But even when they work, they're not "free lunches" either.
When a company designs a vehicle, they tune the engine/transmission to work as a system. They computer tune for a variety of things:
1. Engine performance
2. Engine economy
3. Longevity
4. Ride confort
5. Emissions--both immediate and long term (how it affects the catalytic converter)
5. Etc.
For example, you could accelerate faster if you dumped fuel in the system and the auto-trans went to redline every time before shifting, but it would be a jerky ride with gas mileage, and hard on the system (more repairs / earlier vehicle death). Plus you wouldn't completely burn all the fuel (think re-preogramed diesel pick-ups that get a lot more power, but belch black smoke, like Chipzilla) resulting in a shorter life catalytic converter.
When you buy a reprogramming chip, it has a disclaimer. You will get more power, but will have to sacrifice something else. You can decide if it's worth it to you.
Diesel chip: 100+ more horsepower from a chip...even if they have the dyno to prove it, was everything from the fuel injectors down (pistons, cams, tranny, drivetrain, etc) set up to handle that? Again, my friend's diesel truck has a huge gain in power, but he blows black smoke and I have to wonder what long-term effect it has (besides lower gas mileage). At least no cat to worry about!
http://www.tsperformanceproducts.com/home.aspWayne
