antilag turns your turbo into a jet engine by retarding the timing by 40deg or so every second or third piston stroke (so the motor doesn't stall). this way combustion happens in your exhaust manifold and spins the turbo to see +ve boost at idle. really only necessary for the serious race and rally cars, where you hop off the throttle and onto the brakes and the computer keep the turbo producing boost to eliminate lag once you jump back on the throttle again.

onto the computer issue, and a computer is only as good as it's tuner. run a microtech on it's base program and it'll run like sh1t. have it tuned and it'll run better than a factory computer. install a haltech and run the 'base program' (i inverted that coz most don't come with base programs, you'll have to hunt for the settings to get it to run) and it'll run like sh1t too. have it tuned and it's the same story again.

the trouble is, most people with microtechs install it themselves and use the base program then whinge about it's flat spots and high fuel consumption. 2 hours on the dyno will sort it all out and the difference is phenomenal.