I've seen ads for those before, they look insane and I've always wanted to try one. Sadly I've never known anyone that has had one and I'm not going to spend the money just to try one of those.
I worked with a professional gamer for a while (she was actually a frag doll) and instead of using those things, they did something called the crab grip when playing Halo. I saw her playing like that one day and it's pretty crazy but it makes sense and that's apparently how they all play. It's the same concept as that crazy contraption in that it allows you to use the right analog stick and face buttons at the same time.
You do it by holding the controller with your left hand as normal, but with your right hand, you but your pointer finger on the face buttons and then use your middle finger and ring finger for triggers. It feels really awkward but I guess if you're use to it, it makes you that much better since you can turn and press buttons.
I remember using the "claw" to play halo (we called it the claw instead of crab but same concept) and it does work well once your used to it. This looks like it could work well aloso, but kind of hard to get used too. Maybe i'll gewt on for next gen. I hate buying add ons at the end of a gen.