So... i don't know if Isayama Hajime is solely responsible for writing the story here, but if he is, that's all more impressive for a guy that couldn't draw two decent illustrations consecutively to save his life. Human drama is superb, while characters well developed, complex, and capable of changing as things keep happening to them. Main drawback would be the titan lore, that sometimes feel flimsy and conjured on the spot, just because the plot called for it.
In short, it's good to very good depending on the arc. Don't let attention deficit disorder animufags complaints discourage you. Pacing is similar to Arakawa's Fullmetal Alchemist - there's lot of detours, flashbacks and seemingly disconnected moments, but in the end, it almost always turns out they did have a purpose. Everything clicks. That said, one's probably have to have an appreciation for non-linear narratives to fully enjoy it. In the future, when completed, this series will probably make for a great binge read.
...also, those fake "previews" and "alternate continuity" shorts at the end of each volume are great: