I haven't read the manga, so I can't speak from its behalf, but the anime, oh my god it's hard to compare it to any other existing title.
I've seen a lot of series, and some of my favorite ones are those that have intriguing slightly philosophical or ethical overtones and strong characters. (Berserk, Monster, Gungrave) though I also like series that are just generally well directed and maintain a solid flow (Death Note, TTGL, GTO, Steins;Gate, etc), though if they get way too illogical or incoherent, I find them pretty hard to enjoy. (Hello Code Geass I'm looking at you.)
However, I can't see how Shingeki No Kyojin's anime adaption would have a slow progress. Maybe compared to some titles (which ones, actually?) but I feel that most episodes have succesfully established strong themes. I was pretty amazed how the episode that explained Mikasa's past was done, for example. (The scene which explains natural instincts.)
I keep continuously noticing different thematic motifs and references to modern world. I feel the series presents many interesting topics that have just been disguised to Shingeki no Kyojin's setting. The series also has quite a bit of foreshadowing, so I wouldn't say it's confusing.
I wouldn't say that the progression is entirely predictable either, though, because unlike other animes this series manages to maintain relatively realistic approach, by which I mean that the characters and their actions feel real because their logic and feelings aren't neglected in order for the plot to advance.
Long story short: I feel this series has mature approach and it is thought provoking. If you've got a very short attention span, it might not be fore you.
For me it's currently tying on my favorite series' place with Berserk.