Preferrably on a 4cyl car...

How do you like it? Any real noticable performance gains? Did your gas mileage take a hike south? whats its overall effect on the motor... does it still idle ok? Any other down-falls? Will I be asking for trouble putting more gas into my motor with an aftermarket coil producing more spark? Will I be needing an A/F gauge to get the best results? thanks.