I Went to see Constantine last night, I thought the film was very enjoyable. It's one of those films were you can just switch your brain off for 2 hours and not have to think too strongly about what's going on in the film. The film is about a guy who basically goes around hunting and killing demons and tries to maintain the balance between heaven and hell. The special effects are great and there is enough action to keep you amused.
A few people after the film seemed to think Keanu Reeves acting was a bit dead pan, which I guess it was, but if you had seen as many demons as his character had, I think you would be a little dead pan too, so I don't think it spoilt the film.