Getting more and more aggressive every time they release one of these. This is Robert Pecnik - the owner and designer of Phoenix Fly, a wingsuit company in Slovenia, who has been flying mountains and constantly improving his craft for the past fifteen years or so. Just to give you an idea of why this is particularly impressive, I'll enumerate only some of the many ways his skill is the only thing keeping him alive - jumping off of a cliff notwithstanding.

First off, he is flying directly above stuff. Flying next to stuff is no longer a very big deal because at any time you can just turn away and fly into open airspace if you end up getting in a bad situation. In contrast, if you are above something, your only option is to fly more efficiently and away from the terrain to get to clear airspace. Needless to say, this is more difficult to do than a simple turn and the fact that he intentionally places himself in this situation for almost the entire flight is pretty impressive.

If that wasn't dangerous enough, he is constantly zigzagging back and forth. Basic aerodynamics dictates that when you bank your wings for a turn, a smaller portion of your lift vector is directly countering gravity and you therefore lose lift - which is something he can't spare very much of when he's just above a tree. In order to do this, he has to be very very confident he can keep enough speed throughout his turn so as to not fall down on the mountain and then be very very confident he can have enough left over to continue flying above all the other obstacles he has ahead of him.

And finally, he does all of this in the basement of valleys which give him essentially no room for error. Turn too wide, and you hit the valley wall. Turn too sharp, you hit the valley wall. Turn too inefficiently, you hit the valley floor. Keep in mind that these things handle something like sliding around on ice with shoes rather than ice skates. There is a lot of imprecise drifting involved and it's really hard to turn on a dime (especially without losing lift and hitting the ground).

Oh yea, he's doing all of this at 100 miles per hour.

They'll be releasing a mini documentary in the near future, explaining all the preparation that goes in to making this vaguely safe. I'll post it.