I don't know enough about The Spirit to be able to say whether or not the story was lifted frame by frame from one of the comics (like Sin City was), but I will say that, if it was, it shouldn't have been. You have to figure one thing - A comic, created in the 1940's, that hasn't been popular in fifty some odd years, might need some sort of update to it to make it meaningful to todays audience. The whole thing was idiotic and I just didn't care about it the whole time I was watching it. In fact, the only two interesting things in the film was Scarlett and Eva Mendes (Sorry ladies, but you don't get much eye candy, unless your a fan of both Gabriel Macht AND Samuel L. Jackson).
Honestly, I would say don't waste your time with this one. Of course, by the time of this writing, I don't think its even in theaters anymore, but don't buy it or even rent it on DVD. If its on TV, then watch it, but don't waste your hard earned cash in any other way on The Spirit.