My understanding is they do increase performace-- they're not a gimmick. What I don't know is what (if anything) breaks after you install them. Do they make the coils run hotter, do O2 sensors fail, does gas mileage improve or get worse? The stage 1 (if I'm correct) allows you to continue to use regular gas. The Stage 2 demands you start using premium. So you should calculate the cost in gas expenditure and decide if you really need it. I thought it would be sexy to get one of those but in the end, I decided my Jeep's not a race car, and it actually has very good acceleration and power as it is. Edit: Let's put it this way: I've never been in a situation where I slapped my palm to my forehead and said "Oh, if only I had more power!" My friend with his 87 Toyota pickup with a four-banger and 31" wheels, on the other hand, cries out in pain everytime he goes up a steep hill or enters a headwind. He's convinced he's got a carbeurator problem, though. So we'll see.