Keanu Reeves in a special effects film with Rachel Weisz where he fights the devil? So, what are we talking here: a blend of The Matrix, Chain Reaction and Devil's Advocate?
No, actually it's more like Dogma set in LA instead of New Jersey, with a far larger budget and a lot less chit-chat. Which is not to say that Constantine isn't clever...the hero's initials are JC and the heroine is named ANGELa...definite high-brow stuff, here.
Constantine is based on DC Comics/Vertigo's Hellblazer, and it shows. I've often said you can always tell when a movie was originally based on a stage production because the dialogue takes center stage and the locations and backgrounds become superfluous. It's equally easy to tell when a movie is based on a graphic novel; the scenes are painted with utmost care and the dialogue shrinks to fit inside speech bubbles. Constantine is so finely drawn that you can almost see the white borders between panels.
It's dark. It's amusing. It's also a good anti-smoking public service announcement. Just make sure you stay through all the credits. It's worth it; there's a little postscript that ties up one character's storyline quite nicely.