At times I thought it a bit pretentious and unrealistic. Oskar, the nine-year-old protagonist, annoyed me most of the time. Him being written as a genious nine-year-old simply felt like an excuse not to write a believeable one. In between his geniousness he seems like he could've been a great character, but I do think Foer should have chosen a character with a different age.
At other times however, the book was absolutely heartbreakingly well written. All the parts written by Oskar's grandparents were beautiful, and really made up for the parts that I didn't like.
The typographical experimentation was interesting, especially since it made the book feel more real in someways, and much more like words on a page in others. All in all I thought it successful, even if I felt it was a bit gimmicky in places. Towards the end Foer seemed to become more comfortable with messing with his chosen format, and so it also started working better.
So yeah. I liked it. But if I ever decide to re-read it, I think I'm going to skip all Oskar's parts...