I think that Frank Miller screw up on this one, big time. The basic plot was mediocre and tired. The execution lacked depth. The dialog was snappy at the expense of being meaningful. The characters were mostly cardboard. The pacing was awful. The acting was actually very good, particularly Jackson's. He was awesome, and almost made the movie worth it. But the directing, I really can't give Millar much credit there. The tone of the movie was simply off. It was all style and no substance. I wasn't expecting The Dark Knight Returns, or Sin City.
One part of me thinks that he was trying so hard to imitate Eisner's style that he completely missed what The Spirit was about. And another other part of me reads Millar's Robocop 2 comic and thinks "my god, that really is unfilmable". And I just have to say that Millar really needs a collaborator when he works on films. A good director. Soemone who can cut out the crap and keep the movie on track. When he has that, the result is amazing, liek Sin City. But after The Spirit I do not have any sort of confidence in him making a movie alone, as writer and director.
Millar's worst mistake was getting rid of Ebony White. Okay, the Minstrel style isn't going to fly. Who cares? He doesn't have to keep that style. But he should hav ekept Ebony. He's the heart of the book. Without him, it isn't the Spirit. And having him to play off of would have allowed Millar to give the Spirit some actual depth.
The second mistake was the mystical bullshit. No need for mystical godblood, thank you very much. The Spirit is a hard boiled crime comic first and foremost, with a little by of the fantastic to spice things up. Having the plot revolve around the fantastic was, well, a bad idea. And so was having silly comic-relief clones.
The third mistake was the rating. I suppose that the studio might be to blame, but so migh Millar. Really, they should have gone for a hard R. Eisner wrote The Spirit as an adult comic, as adult as anything could be in the 40s. If he would making the movie, he probably would have shot for the R.
And really, all the sucktastic dialog and the horrible pacing and the failure to flesh out the characters were mistakes, too.
Ultimately, Millar kept The Spirit's style while discarding it's core values and themes (perhaps unintentionally), to the point that the movie is more of a parody than an adaptation.
If you want to watch a sucktastic movie in the style of a 1940s Eisner comic it is alright. If you want to watch a faithful adaptation of The Spirit, you're SOL.