It's not garbage (although the novel is better). I don't know why people hate it so much.
It actually follows the light novel very closely and I actually wish the anime was more like it, even though I guess that would have been redundant in some ways. It IS different in presentation from the anime though. While the basic story is somewhat the same, the events are very different in the manga than in the anime. Some of the characters are a little different too. Mikado? The Yagiris? Mika? Izaya? The Van Gang? Yeesh.
One thing you'll most likely be surprised about is how the manga begins. Let's just say that the anime probably changed the introduction for the better, because if they went with that intro, some of the mystery would have been killed.
A story shouldn't just give people what they want, but also what they didn't know they wanted.