Horner did nothing special for me for TASM1. The only tracks I liked from his score were "The Briefcase" and "Rooftop Kiss" (they felt quite reminiscent of Elfman's work) but "The Briefcase" ripped off Zimmer's "One Simple Idea" for Inception which is disappointing.
It's pretty cool, and quite the coincidence, that Elfman did a great job on Burton's Batman and Raimi's Spider-Man and now we have Zimmer having done both Nolan's Batman and Webb's Spider-Man. Elfman did a great job at creating a memorable theme for Spider-Man and though I think it will be hard for a composer to top that, for Spider-Man specifically, Zimmer did an excellent job at creating a grand, superhero theme for Spidey this time round (I smiled when I heard it in the opening Spidey scene and when I believed it was used again at the end of the film as the credits rolled). If Horner had returned, I have a feeling I wouldn't have enjoyed the film as much because music plays such a huge part in creating the right mood and tone that you want to convey in a film and Zimmer got that just right.
I'm not sure if it's possible but I'd love to see an edit of TASM1 but with hints of Zimmer's score (and other pieces of music from different composers) in the appropriate places because, frankly, Horner's score was quite a disappointment to me; it didn't really fit or do anything special for Spider-Man.
I'm not too big on dub step and was a bit wary when I first heard it in the film alongside Electro on screen but quickly accepted it and that it fit Electro. With Electro gone though and most likely not returning in TASM3, there should hopefully be no need for dub step in the sequel's score.