One detail which bothered me the most, was Penguin's plan to build that giant bird-lure-rocket for terrorizing the city because in that way he'd leave no fingerprints or traceable evidence. That was one of the dumbest explanations I've ever read in a comic book, at least it's 60's quality of writing. I would give the flashbacks and built-up also a 4/5 but the conclusion's heading to a 1/5. Quite a shame. As if they switched writers in mid-process. Or they didn't know what to do with their strong premise and just wrote it anyway.
Oh, and the Joker cameos were brilliant!