I honestly think that more people would be okay with Miles if he was introduced before Peter died or if Peter was still alive and powerless and served as a mentor to him, like Bruce and Terry. A lot of the anger towards him is based on race, and some people dont like him because Peter died and he was replaced so quickly. I think more fans wanted some kind of closure (which is what I was hoping Spider-Men would do) but they didn't get that.
I really don't like it when people say "You can just read 616 Peter." 616 Peter and Ultimate Peter are two different versions of the same person. They're only similar on the surface, It would be like telling a Power Girl fan "You can just read Supergirl, they're both Kara Zor-El." Most PG fans didn't know if we were going to see her again before World's finest, and even after people are still angry.
Personally speaking, I wanted to like Miles. I thought that a Spider-Man who looked like me was a really cool thing. Its a shame that we have to justify his existence because he's a black male lead, most of the time its just Black males, not women or other minorities.
After being skeptical at first, because I liked Ultimate Peter too, I read the first trade with Miles, since Bendis's work reads better in trade. I didn't think that it was bad, but it didn't wow me. I really wanted to jump on the Miles fan train, but its just not happening for me, and that's fine. I still think its good that he has a book, even though I'm not into it.