This is, essentially, a murder mystery set in the DC universe (which is appropriate, I suppose, for DETECTIVE Comics). For reasons of continuity and sexism, it is obviously necessary that the murder victim be someone's wife, and the wife of a Z-list superhero at that, but in spite of the absolute tiredness of this trope, some of the strongest material in this miniseries is at the beginning, where the Elongated Man talks about how he met his wife and their marriage; it is genuinely touching, and sufficient to make the murder of a character you've never heard of before seem sad.
The story of the subsequent investigation mostly pushes Supes and Bats into the background, for sound reasons: they tend to dominate any story they have much of any part of. However, this means the story is dominated by what seems like scores of characters that I find difficult to tell apart and whose costumes and powers seem very old-fashioned to me. The middle issues devolve into a confusing muddle, with clues doled out by a mysteriously glacial autopsy process directing the otherwise directionless investigation. In a story about a (presumably) non-reversible death, it seems particularly bizarre and inappropriate that various characters talk openly about "when [currently living character] was dead", like this is just a thing that happens. The scene where someone talks to the ghost of a dead Green Lantern about when he was going to come back again was particularly off-putting to me; maybe this is a Green Lantern thing that I'm not getting, but if so I'm not a fan.
The final twist and solution is adequate, if nothing special for a murder mystery; a solid C+, shall we say. There are issues raised in a plot thread that are never adequately resolved; I suppose this is what is meant by "ripple effects throughout the DC Universe for many years to come", although I have no idea where I would pursue them and no real inclination to do so. In conclusion, Joss Whedon enjoyed this story a lot more than I did.