It's highly praised. Christopher Nolan and David S. Goyer discuss it in the graphic novel's introduction, explaining why it's the biggest influence on their Batman movies.
I didn't think it was great, to be honest.
For me, its problem is that its story parts aren't integrated. Someone's killing people on national holidays, so they call him "Holiday". It's a serial killer story, then, and its heroes are detectives (Batman and Gordon) and district attorneys (Harvey Dent)... but no one actually does any detecting. They just bitch at each other while Holiday shoots people for a year. (Oh, and he mostly kills gangsters who were asking for it and would probably have ended up killing each other anyway. My level of involvement was... uh, not the highest.) One assumes that detective work is going on somewhere, but it's offscreen. As a result, I didn't care who Holiday was, since we'd been given no clues and it could have been anyone. Is it A, B, C or D? Well, it could be any of those, or indeed some random loon we've not met yet. The answer also wouldn't change anything. It's arbitrary and could be whoever the script wanted it to be.
This also makes our heroes ineffectual. The only memorable thing Batman does is get hypnotised by Poison Ivy and tell Gotham City Bank to get into bed with the city's biggest gangster. (Well, I tell a lie. He also brings a thanksgiving dinner to a zombie. His dangerous, flirty scenes with Catwoman do nothing for me, though, because for me Catwoman was, dramatically, dead space. I'm not sure the character really has a point, at least here. She's sexy and enigmatic. That's it. She's there to tease Batman and the reader, but I never believed that she was ever going to change or make a difference to anything.)
Meanwhile, the only memorable thing Gordon does is to take so much overtime that you're slightly impressed that his wife stays with him, although it helps that they have a child. He's in the story to be the upstanding moral voice of the Batman-Dent-Gordon triangle and to be Barbara's husband.
As for Harvey Dent... well, his marriage is similarly challenging (albeit with significant points of difference), but of course we're aware of his fate as Two-Face. This is done... and it's okay. Nothing wrong with it. It's a perfectly respectable retelling of his origin story, but I wouldn't call it anything special either. It gets weight mostly from the series's serious tone and sheer page count.
Oh, and the dialogue doesn't sparkle either. There are moments (e.g. Catwoman attacking Poison Ivy) where you'll wish the page had been silent. The art's spectacular and the line's fine in principle, but someone should have looked at the finished product and said "these don't go together". An editor, for instance.
That said, though, there's good here too. People have raved about this series, after all.
I'm a fan of Tim Sale's art. It's very film noir, a bit Sin City. (Well, that's what it sometimes reminded me of, anyway.) Lots of shadow, lots of bold compositions and character designs. I'm not convinced that Sale's Joker is human, for instance. (Look at those teeth.) He also draws such a cool Batman that you almost don't mind that Batman's essentially a bystander to a domestic mob crime epic.
It's set in Frank Miller's Year One continuity and follows the Roman and his criminal empire. There's nothing comic-bookish about these people, except maybe for Sofia Falcone Gigante's character design. They could have slithered in here from a Martin Scorsese film, or The Sopranos. They order gang hits and get shot themselves. If this series is about anything, it's about these killers, their connections and their families in a world of criss-crossing murder. They're the real protagonists, to be honest. They're the ones who are doing stuff. The domestic problems of Gordon and Dent are a counterpoint to theirs, not the other way around. The art style's a perfect fit for them. From that point of view, the series works.
The Roman starts hiring Batman villains rather than regular gangsters, which annoys his sister and eventually sets up a sort of transition from the realistic gangster world of Year One to the more exotic Batman villains we're familiar with.
I also liked the portraits of Gordon and Dent's wives and marriages. It's well done and a bit unusual for a Batman book.
In fairness, this isn't the end of Loeb and Sale's story. It has two sequels. There's Batman: Dark Victory (which I've bought) and Catwoman: When in Rome (which I don't expect to bother with). I'm mildly curious about how this continues with Dark Victory. Also, with hindsight, the pieces of this story's mystery fit together neatly (e.g. the out-of-pattern murder of the coroner). I'm not calling this series bad as such. It kept me reading. I can see why it grabbed people. It's Very Serious, in that way that fans of sometimes-childish things will often yearn for. I see why it resonated with Christopher Nolan.
For me, though, it feels like half a story. The baddies' story is ambitious and could almost be called epic. Two-Face's origin story is in there too and is being given due weight. The heroes, though, feel like an appendage.