Exceptional show, and as a whole the series has an absolutely top notch storyline with almost flawless continuity, along with superb animation and a very clean art style backed by an excellent soundtrack.
I'm not usually into manga, but enjoyed this series very much. The 1st season in particular is eery, dark and haunting, giving a feeling of utter bleak hopelessness in a world where humans are on the brink of extinction.
One of the largest shames for me were Eren's long, drawn out, repetative, childish, whiny rants, along with his bratty personality (though i think a lot of that was the tones the voice actor for the dub version used). Also I felt some of the main characters were somewhat hollow, having an unforfilled, incomplete personality, Mikasa being the character whom I thought this stood out the most.
EDIT: I would highly reccomend watching the OVA episodes after finishing the 1st season. Though episode 8 in the OVA's (Lost Girls: Lost in the Cruel World) should be watched after finishing season 2.