You've seen this type of movie before, because it has been done so many times before, with "Ghost in the Machine", "The First Power", and "Wes Craven's Shocker" coming to mind. Typically, in the 'detective vs. demon' horror subgenre, the story follows the same predictable pattern:
"Fallen" is an extremely boring by-the-numbers exercise of the above that brings nothing new to the subgenre. John Hobbes (Denzel Washington) is first seen stumbling through a snow-covered forest, and the voice-over makes mention to 'the time I almost died'. The story then jumps back to the execution of convicted serial killer Edgar Reese (Elias Koteas, the DJ in "Exotica"). Minutes before the execution, Reese begins singing the old Rolling Stones tune "Time is on My Side", which becomes a recurring motif in the story. The chamber fills up with gas and Reese expires, ending a hard-fought battle for justice by Hobbes and his partner Jonesy (John Goodman).
The next day, Hobbes and Jonesy return to their regular routine as homicide cops, putting the Reese case behind them. However, they are soon called to investigate a series of murders that duplicate the Reese killings, but seemingly carried out by different individuals. The cryptic clues left by this supposed copycat killer lead Hobbes to a cop suicide that occurred thirty years prior, and while digging for clues, he comes across the dead cop's daughter, Greta (Embeth Davidtz), a theology professor at the local university (isn't that con-ven-ient?). Together, they must unravel the diabolical scheme of the supposedly-dead Reese while an unknown force is gathering evidence against Hobbes, linking him to the recent spate of murders, which is eagerly pursued by Hobbes' superior, Lt. Stanton (Donald Sutherland).
"Fallen" is a miserable excuse for a horror movie. The story lacks anything remotely resembling suspense, there were absolutely no edge-of-your-seat-scary-moments (actually, there was one mildly scary scene, but it was over pretty quick), the characters were flat and uninteresting, and the movie is burdened with too much narrative baggage, most notably the incessant voice-over by Hobbes, which is neither eloquent nor profound. Director Gregory Hoblit also ignores the basics of story-telling economy, by lingering too long in many of the scenes, or padding the movie with unnecessary exposition that does not propel the story forward. Though the movie is highlighted by some ostentatious visuals and the half-decent payoff of a twist-ending (though some astute viewers will be able to figure it out ahead of time), "Fallen" is just another in a long line of horror movies that attempts to cash-in on yet another creatively-bankrupt paradigm that should have died out long ago.
I don't know who would enjoy this movie. Maybe I'm just jaded because I've seen four other movies in my life that follow the same predictable pattern. If you are curious, wait for the video. But if you have seen at least one movie that begins with a serial killer being executed, then you'll probably want to pass on this one.