It's not perfect, but I think it's the better of Ennis's two Batman stories (that I know of, to date). It's reuniting him with his Hellblazer artist, Will Simpson, and it's much more within Ennis's comfort zone. No superpowers. No magic or fantasy. A pair of bounty hunters, one an ex-cop and the other a Vietnam veteran. This is mostly a street-level Garth Ennis story that happens to be taking place in Gotham... but the exception to that is Doctor Freak, who's what you'd expect of a Batman villain and his drugs couldn't be called realistic.
I liked this. I think it's a good way to use Batman. References are to the real world, not to other DC heroes. (That ex-cop quit after the Rodney King beating, for instance.) That said, though, the story's looking back to the 1960s, pitting a Vietnam veteran against what's basically an evil hippie John Lennon. (Batman doesn't really like either of them, but he can tell who's an anti-hero and who's an out-and-out villain. "Enough of this hippie garbage!")
There's something a bit odd about Simpson's art. There always has been. It's clean and easy to read, but I'm not convinced that his visual storytelling is the strongest. Issue #91 had me wondering if some pages had been printed in the wrong order, while it's hard to follow the action in the issue #93 finale. (For what it's worth, Simpson became a film and TV storyboard artist not long after this, including for Game of Thrones.)
I'm sure both Ennis and Simpson would look back today and wince at the lack of storytelling clarity. That said, though, I enjoyed this story. It has strong dialogue, good pace and a disgusting bit at the end of issue #92. It fits well into Legends of the Dark Knight (a creator-driven series that I like a good deal). I don't know if it's strong enough for a graphic novel and I bought it as individual issues, but it works both as a Batman story and as a recognisably Ennis one. It's worth a look.