My verdict: I thought this film was okay. The CGI is a little primitive in spots, but not to the point of being distracting. Shia LaBeouf's character was completely extraneous but the rest of the supporting characters were awesome (and getting Peter Stormare to play Satan was a brilliant stroke, even though it probably wasn't an obvious choice). There was one plot twist which caught me by surprise, which was pleasing.
It is implied that Catholicism is the true Church, but the theology isn't entirely consistent with the Catholic Church. That didn't bother me, but the flawed theology that the filmmakers chose to use instead did. In some places this invented theology is sound (e.g. Constantine is told that doing things to get into heaven won't get you into heaven—doing them because they're the right thing to do will get you into heaven) in others it is not (e.g. there is a ridiculous stalemate between God and the devil that neither can violate; they can only prevail against each other by influencing human choice ). Also, having a female angel named Gabriel was incongruous (even if they did try to make her look androgynous). It was nice, though, that it was consistent. This allowed them to use the rules between God and Satan to develop the plot. The way Constantine tricks Satan (and technically, God) the first time is clever; the second time is too obvious to believe that Satan would fall for it. This does, however, give a satisfying conclusion to one of Constantine's character flaws.
DISCLAIMER: I watched this film as edited for television. I have not seen the original R-rated version and cannot comment on its appropriateness.
 See, for example, this one.
 If you wonder why this is unsound, allow me to explain: the real Satan only has what power God allows him. Eventually God will defeat him—regardless of the impact of human choice—and exile him to Outer Darkness. No matter how many souls Satan wins to his side, no matter how many atrocities he convinces human beings to commit on Earth, God will still win. There is no stalemate. And Satan is going to lose no matter what.
 For more info, I suggest you consult the reviews at http://www.kids-in-mind.com/c/constantine.htm and http://www.screenit.com/movies/2005/constantine.html.
Sculpture of an angel is by Jotquadrat, available at http://commons.wikimedia.org/wiki/File:Brenig Evergislus Engel.jpg.
Sculpture of a demon is by Zarateman, available at http://commons.wikimedia.org/wiki/File:Vitoria - Catedral Nueva 13.JPG.