I tried, but the problem is that the movie actually gets pretty close to the comic in some key places. Setting aside the fact that he's American and has dark hair, Keanu Reeves actually makes a pretty good John Constantine. The attitude, the way he carries himself, the way he lights a cigarette... the character is there and intact. Contrary to some early, troubling production art that got passed around the Internet, there is no Constantinemobile and John doesn't drive at all. And, like in the comic, he's taxied around by a cabbie named Chaz (although the movie version is a much more enthusiastic sidekick than the comics version. There's also an attempt to work in Garth Ennis' famous "Dangerous Habits" storyline.
So, as I'm watching the movie, trying to pretend it's about someone else, I keep getting reminded that, no, this is a Hellblazer adaptation. To judge it purely as something else would mean that I'd have to ignore some things I liked about it, like how Keanu lights a cigarette. But if I let myself count those things into my final estimation, I also have to count stuff that I didn't like, like the crucifix-gun, the magic tatoos, and John's switching to gum.
And seriously, I doubt I'd have liked the crucifix-gun no matter what the name was of the guy carrying it. Really, Constantine is a pretty average monster-hunter flick with decent special effects. It's only in comparison with Hellblazer fans' original expectations of it that it looks good.