My GF and I both *love* Constantine. It was great in the theater, and it was great at home on DVD. Re: the "more about struggle between God and Satan", I think that they would cover more of that in sequels which it seems unlikely will be made.
<spoiler alert!> I thought that the principal plot of this movie was the fall of Gabriel, and I thought it was told well in its oblique way.</spoiler alert!>
I have now seen it 3 times, and each time I see more how every piece of the movie goes to support that. Apparently they shot another entire sub-plot about a love interest for the title character. While I would like to see it (is this on the extras on DVD? we haven't explored them yet), I can see why they thought it would distract from the tight focus of purpose that the theatrical version has. Going outside of that focus to include more of the larger good/evil struggle would have (IMHO) been as distracting as a love-interest story line.