I've always used one..on the 4 Iand 4s I used one from ifrogz... So far the for the 5 I got one from bodyguardz... It works great... It's 4-way too...of course it takes away from the screen quality... But if privacy is important to you... It's worth it. I have a removable one from 3m on my MacBook air too...I just can't stand people staring at what I'm doing... With the bodyguardz privacy screen I haven't experienced any degradation in touch... Sometimes the screens are so thick your gestures on the phone aren't detected.
In any case if you do want one...I recommend buying the black iPhone..l the privacy screen blends in with the phone...people won't even know you have one on...people always ask me if my phone is even on... If you have the white iPhone it looks kind of stupid with a black screen on the front... Because you can still see some of the white body neither the home button and near the FaceTime camera.