The premise of the anime is that huge humanoid creatures called Titans almost drove humanity to the brink of extinction several 100 years ago. The only way humanity was able to survive was to build huge walls and live inside them. Very interesting stuff, and the story builds up nicely too. Since humanity is mostly in the dark about the titans, we as a viewer are subjected to the same treatment and slow reveals of new info as the plot about them as the plot moves forward is always exciting and shocking. The atmosphere is very grim and desperate. Sadness, hopelessness, loss etc is always looming overhead, perfect for this type of premise.
This anime has an extremely EPIC!! feel to it. The soundtrack, the speeches, the motivational talk, and of course the action, all have the highest level of epicness applied to them. Infact, the word 'Epic' falls short to describing it. And this is the strength of the anime. Underneath all this epic layer, you will find a large set of interesting characters, subplots and conflicts, all of which are far from being the best but are good enough. The protagonist is a good mix of shonen ingredients with nothing in excessive quantity. Its the EPIC layer over it which amplifies everything and even the simple stuff becomes awesome.
The animation quality is superb, one of the best I have seen so far in an anime. The environment, huge fights and fast movements of the characters using there gears is beautifully imagined and rendered. So pretty!! The action was OH SO SATISFYING. I can't express how awesome it was, extremely adrenaline fueled and impactful.
Soundtrack, like I mentioned earlier, is epic. Loud orchestral pieces like the ones you gear in end of the world type videos really amplify the onscreen action.
On the downsides, it sometimes abruptly shifted tones, which was somewhat distracting. Also, some of the later episodes had slower pacing, which normally wouldn't have been a bother but seeing how explosive the earlier episodes were contrasted the later slower ones.
Overall, an amazing EPIC anime. One of the best I have seen. I haven't this much fun/enjoyment watching an anime since Fate/Zero (Another epic anime). If you still haven't watched this anime, stop everything and watch it asap, if nothing else you'll atleast enjoy the epicness. HIGHLY RECOMMENDED!!