Disowned by it's creator, Alan Moore, this is the movie of the comic book Hellblazer. I could talk to you for hours about the masterwork that is the book, but that's not what you're here for, so lets talk film.
Constantine is a small sojourn into the life of one John Constantine (Keanu Reeves, as I'm sure you all know), exorcist for hire. As it happens, this visit happens to be whilst he's following leads hinting that the Apocalypse is, as they say, nigh. Rachel Weisz plays an L.A. cop, searching for a reason behind the suicide of her sister. They soon find that they're on the same trail, and start to work together.
Rewritten as an american to hook producers (the original script with a liverpudlian John met no interest, but an exorcist from L.A. got executives excited - go figure), John Constantine retains some of his attitude from the books, but little else. Already judged for committing suicuide when a teenager, he knows he's going to hell, and has spent his life attempting to atone by sending the agents of evil back home when they break "the rules" of God and Lucifer's bet. It's a backstory which creates a very similar character, in a very different situation.
Actually, I am going to tell you a little about the book, to highlight the differences. The film is mostly based (very, very loosely) on the storyline from 'Dangerous Habits'. In Hellblazer, Constantine is heading to hell because of the life he's led - dabbling in the black arts, attacking his father, breaking commandments left right and centre. A large part of the books is involved with him atoning for his life, but not for God - he is trying to fix what he's brought on his friends. Before 'Dangerous Habits', the DC Vertigo universe has progressed (in Neil Gaiman's excellent Sandman) to Lucifer quitting his enforced job of running hell, and a triumvirate of demons now do the work. Constantine has discovered that after the reckless, dangerous life he's lived, he's going to die now, and not of some grudge or plot, but of lung cancer.
In contrast, the film takes a more traditional religeous stance. When Constantine goes to Gabriel to ask why his fate is as it is in the books, he finds it (angels have no gender) in its club, where it influences world politics. In the film, Gabriel is the head of a church. The conversation between the two is brilliant, and very similar to the book, on later examination. Lucifer is still very much in control down south, and demons are after religeous relics to bring about judgement day.
The film is fantastic - I enjoyed it a lot, and I'm not going to deny that. However, I also think that Moore was right to disown it, due to the fact that it doesn't portray the book. It's a rip-off, just one that credits the original. Chaz, one of John's oldest friends who ferries him around London in a cab because he owes John, is now his young apprentice - bumbling and ambitious, but reliable in a tight spot. Contantine is more calm and collected, losing some of his rage at life and God's system. Three of Constantine's friends have been amalgamated into 'Father Hennessy', who is actually the most believable DC Vertigo character in the film. There are moments of self-congratulation and sensationalism in the film that the books managed to avoid throughout, embodied by the holy shotgun pictured with Constantine in posters. It's as if the director said to Moore "Well, it's ok, but it could use a lot of work". Which, effectively he did, as he is quoted in an Empire review saying he took the bits he liked of the books and then made 'his film'.
The effects used in the film rarely take away from it's believability, with the exception of some of the demons. One demon made of insecs and crustaceans is really, really cool, but is very obviously computer-generated, cutting away from realism. Also, the demons who populate hell are all identical to each other, with the same wounds and contusions. A scene which is beautifully done is the torture and murder of Father Hennessy, with the most subtle of effects adding great power to the frantic desperation of the victim.
There are some painful hollywood cliches, like feeling the need to wrap up a relationship between a male and female character as if it had been a love story, but manages to redeem itsself somewhat by avoiding a truly cheezy ending.
As a horror/thriller, this is a great film, with the special effects rarely interrupting the scaryness of situations. Despite early criticism, Reeves plays his role fantastically, as a downtrodden, end of the line, cynic who knows more than he'd like to, and Weisz's hard-line cop caught in a world she never knew was just behind the corner of her eye is convincing. However, as a film supposedly based on Alan Moore's Hellblazer, it is a dimal offering, more the wayward child than a true successor. Which, I suppose, is very apt.