It's not really a stretch to say that "Brand New Day" was controversial and it certainly had its share of detractors, but the majority of them seemed to be directed less at "BND" and more at the storyline that immediately preceded it, and on that front, I'm totally with 'em. But the end result, and I say this as someone with a documented love of the character, is that for the past two and a half years, "Brand New Day" gave us some of the best Spider-Man stories in decades.
I'll admit that at the start, I was extremely skeptical. It was, after all, spinning out of "One More Day," a story that doesn't have a lick of good in it. Everybody and their brother on the Comics Internet has thrashed that thing to the point where it barely even needs to be addressed at this point, but the fact of the matter is that it was a heavy-handed retcon that that was predicated on something completely antithetical to the spirit of the character.
Even worse, the whole thing -- Both "One More" and "Brand New" -- seemed to be motivated by the most pervasive enemy of good storytelling in comics today, nostalgia. The desire as I understood it back then was to return Spider-Man to the heights the character had seen in the '70s, and while those stories were great, I was worried that it was going to end up in the same self-indulgent repetition that often comes with alleged returns to greatness.
With Spider-Man, though, the creators understood that it wasn't just the characters in those stories that made them great, but how they were told. Instead of falling into the nostalgia trap, the creators and editors of Amazing Spider-Man got it in ways that other retcons completely missed.
So whether or not the initial motivation was to just get rid of Mary Jane and the marriage, "Brand New Day" really did give the books a new sense of energy and forward momentum. It's ironic, considering that the whole thing came as a result of an event that was criticized for wiping away years of character development, but for the first time in a long time, Spider-Man stories felt like they had some forward momentum to them.