Warning: This is not likely to be a very fair review.
A take on the Hellblazer comics, the plot is not earth shattering; we've seen the devil or someone like him trying to take over the world before. Keanu moving between worlds, trying to save people - we've seen that before too, haven't we? That said, Constantine was thoroughly enjoyable a movie - for various reasons.
Reason #1: Mr. Reeves.
Keanu , ah well, he is such a treat to look at, so I really wouldn't care a damn about how convincing an actor he is. He is an action star, and a good one at that. But he is not quite the sour, wise-cracking John Constantine who is hoping to buy himself some space in Heaven. Like I said, he is good eye-candy, so I am not complaining.
Reason #2: An androgynous Gabriel, a really wicked Lucifer
Tilda Swinton as Gabriel is a scream - she should have had some more screen time. How tall is the lady, by the way? Peter Stormare as Lucifer is deliciously wicked. IMHO, the best appearance in the movie. I really liked the way these characters have been fleshed out. (BTW, more than half the people in the theatre had no clue who these two were. Alarming!)
Reason #3: Gavin Rossdale
I nearly fell out of my seat when I glimpsed Gavin Rossdale as Balthazar in a pin-striped suit. Yummilicious! Another one who should have got more screen time. Great eye-candy.
Reason #4: Execution
The execution is slick and the fast paced; the first exorcism is simply superb. The action scenes are well done, the insect-demon that attacks Constantine is great. But the second half gets a little tedious, and the special effects in hell are sadly unimaginative. Could have done with some better work on the demons. Loved the angel wings.
Verdict: Must watch. Unhook your brains and enjoy the ride.
(Warned you it wouldn't be fair!)