"Shingeki no Kyojin" (or officially "Attack on Titan", which doesn't make sense) is a shounen action anime. The story places as in future, where humanity has been reduced to medieval level by appearance of non-sentient, human-eating humanoid giants called Titans (imagine giant nude zombies). Humanity is currently behind tall walls place in several concentric rings, enjoying relative safety for last hundred years. It all changes with appearance of a Colossal Titan and an Armored Titan, that together are able to penetrate the walls at one place and release other Titans at people. During this fatal defeat, we are focused Eren, Mikasa and Armin, three kids living in town where the breach happens. After managing to stay alive, but witnessing Eren's mother being eaten alive, Eren vows to slay all the Titans. This brings the three of them to join the training for the Survey Corps, only people that still fight against Titans on their own territory...
My first impression was that good, because I like vivid and strong colors in anime. Next I thought how the anime was a bit over the top with imagination and not very consistent (flesh-eating giants, 100 years of solitude). I was especially off-put with the claim about walls being set in circle with 250km diameter, when you could obviously see the other side of the wall! But then things started to unravel slowly, and I was hooked.
So, even though "Shingeki no Kyojin" is primary an action anime, it has a large dose of mystery. And not the type where character finds solutions which no one would ever realize, they solve them gradually by getting more information. Also, the setting reveals very gradually (there are some info-dumps, but not too clumsy), so almost every episode you learn something new. There was a few surprising reveals (especially the last one!), but they were done nicely and consistently. What I did dislike were too long intros and recaps of last episode at the start of each.
There is really a bunch of characters and you will sometimes have trouble to know who is who. They are typically shounen, so don't expect miracles, but they are pretty solid and interesting. The good thing is that they grow much during the show. I really liked how the author was not afraid to kill them.
And this brings us to the atmosphere of "Shingeki no Kyojin", which is the best part of it. This is a very brutal anime, with lots of tragedy, death and sorrow. I didn't expect so much pain in shounen anime. There are nice changes between fast action pace and slow reminiscence and melancholy. It is compared much to "Claymore" - I agree with this comparison, even though the anime is original enough. Action is pretty good, if you can suspend your sense of mechanic. There is even some comedy, and pretty good one (I really liked Sasha).
I've read some comments about this show having horrible design and animation, but I disagree. I admit that it is simple and not beautiful, but I think it goes great with the atmosphere. I really like the discrepancy between Titan's look and behavior. Voice acting and sound were OK, but OPs were terrible. I didn't even try to listen EDs.
All in all, I really enjoyed "Shingeki no Kyojin". It simple enough for shounen fans, but with enough details for other to enjoy, with great dark and violent atmosphere and lots of mystery. I can't wait for the next season.