If you're a complete driving simulator novice, or you're brought up on a diet of Mario Kart or Ridge Racer, the chances are your braking distance judgment is non-existent. If you're a GT vet, you should be OK. AB takes braking away from the game. When you approach a corner, the car will slow down for you at the same rate as the other cars in the race. If you're racing at a harder Opponent AI level, the brakes will come on later.

The braking is optimized for the racing line, so if you're going wide into a corner, it won't compensate for you. You can over-ride AB if you want and just brake for yourself if you find that it doesn't use enough. Also AB doesn't necessarily use the most efficient braking points. You can out-brake you opponents without it, so it's worth learning not to rely on it too much. It just eases you into the game by taking away the responsibility of braking.