Been there many times, and been to other clubs in Berlin too. Berghain is very special to me with it's one-of-a-kind dark vibe and always an amazing music and people losing themselves in their very synchronized, rough dance movement specific to Berghain. I'm very much into their unique music & quality sound which gets me to more and more think about not wasting my time in other places and just going only to Berghain - quality over quantity.
When you get in, you realize exactly why they have such a strict door policy, and you appreciate it, lots, believe me! - this club gets it's unique vibe mostly because of it's people (and the music - did I mention that already?) and they are like no other. Compared to KitKat where you see a lot of young (and older) wannabes, in Berghain, you see mostly a bit older people who literally belong there and don't act out or try to be someone else.
And now to all those people whining in here about "not getting in without any reason". There's a perfect reason for this and the securities are well trained to feel your vibe, You coming to whine in this forum after not getting into Berghain shows very well you are not part of the right crowd and once again, let's appreciate the hard job done right by the guys at the door for not letting you = the wrong people in. Berghain is not the place for whiners.
Also, for the people who are lucky and get in - please appreciate the vibe and read forums before you come about what to wear inside and don't hold back on black. Cause if you manage to get in with blue jeans and T-shirt, you just killed a piece of this club. So please respect the place and regulars trying to build up the unique vibe with what they are wearing and dress accordingly. And less is more - don't be afraid to wear less, no one cares and you can feel the unique freedom.