For an FPS, I think I'd rather have something like a double-speed turn button instead. It wouldn't work with dual analog control schemes, though.
180 turn buttons are jarring in an FPS or close-camera third-person game. If they turn instantly, then you still have to adjust to the completely new view before you can safely start moving again. (Just because you know you want to turn around doesn't mean that you want to turn 180 degrees.) If it is a slow turn, then it is little better than a manual turn, and potentially worse if you are locked into the turn.
Yes, you can double-tap a 180 turn button for a quick "rear view" effect, but that mucks with your movement as it is a real turn. If you only want it to take peeks, then you probably want the rear view button of a driving game instead.
Now, a rear view that could be converted into a fast turn as desired could have some real potential. Being able to look behind as you continue to move forward, with the ability to auto-turn to the new view when and if desired? (Of course then people would probably then just ask for separate view, aim, and movement controls, so you could do stuff like strafe left while looking right and firing behind you.)
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