Just started watching this, I don't know the source material (except the first 3 chapters just to compare) but so far I'm really liking it. Sakura does come off as naive, maybe even stupid. However, I can deal with her since Oogami is stealing the show.
I'm trying not to get my expectations too high, since buildup like these tend to have disappointing conclusions, but the show is doing a good job at drawing interest.
I'll keep following this and see if this can continue to amuse me by either introducing interesting characters (kinda like Getbackers), or has a strong enough story to prevent it from getting boring.