Critical hits are useful all game with about equal usefulness.
Critical hits aren't as critical to game play in this game compared to other games. That's due to the relatively low change of a critical hit, the fact that critical hits have a very minimal damage, and don't have game breaking high damages either. Still useful, but just not as much as in other games. I -think- that's by choice, but it does feel like critical hits are an undeveloped part of game mechanics.
Nothing quite like doing 2 damage on a critical hit, that's flanking, too.