Spider-Man: Brand New Day started in 2008. I know it’s been over a year and a half but I think it’s safe to give an unprejudiced opinion now.
Just as a recap, Brand New Day marks the change in Spider-man’s status quo where:
·         his marriage to Mary Jane Watson has been erased magically in a deal with Mephisto to save Aunt May’s life (and now Aunt May volunteers in a homeless shelter).
·         The same deal also undid the organic webshooters and spider-spike and returned the mechanical webshooters to Peter.
·         That deal also removed/altered people’s memories so no one knows that Peter Parker is Spider-man.
·         Harry Osborn is alive.
From what I understand, the reason why this was done was to rejuvenate the character. A married Peter Parker is an old Peter Parker. By getting rid of his wife and bringing back his best friend we can get the great Spider-Man comics that we all grew up with twenty years ago (although that’s when I started reading Spider-Man). I would have to say that Brand New Day is a failure.
Don’t get me wrong, I’m not saying it’s a failure because I’m tremendously attached to reading Peter married to MJ. I’m saying it’s a failure because it is a ham-fisted way of allowing the events of Dark Reign to occur.
Dark Reign is the current Marvel event that has Norman Osborn (aka the Green Goblin) as the Director of SHIELD (renamed to HAMMER). Since Gobby and Spidey have a history of knowing each other’s identities, one would have to assume that since Norman has the tremendous power of HAMMER available to him that Spidey wouldn’t stand a chance.
By making the deal with Mephisto so long ago, Peter has saved himself from instant slaughter. In the meantime, readers are supplied with flimsy stories that lead to questions that aren’t being answered or just answered poorly.

Questions like:
What happened to the last twenty years of stories while Peter and Mary Jane were married?
They all still happened, except Pete and MJ were just living together and when they finished making the deal with Mephisto, MJ ran away to Hollywood.
Does Peter remember that he made a deal with the devil?
He makes hints at it (Amazing Spider-Man #596)

Oh yeah... we did have that three-way with the dude in the red leotard.

… but he seems pretty oblivious to it when magic users accuse him of it (New Avengers #54).

Can I have your cake?

Does Mary Jane remember that Peter made a deal with the devil?
She hints at it… but she’s in Hollywood so it’s not clear.
How does it work that people can’t remember Spider-Man’s identity?
Well, the magic somehow blocks a part of the memory so people don’t want to know who Spider-Man is. Even if there is a tonne of evidence saying that Peter Parker is Spider-Man they will put two and two together and accuse Peter’s roommate of being the wall-crawler (Amazing Spider-Man #565). When Peter shows his face to people by removing his mask, all memories are to flood in (as was revealed with the Fantastic Four in Amazing Spider-Man #591). However, Jessica Jones was very surprised that Peter Parker was Spider-Man and admitted to having a crush on him in highschool (as per New Avengers #51) even though she knew his identity earlier.
How is it possible that Harry Osborn is alive?
Shortly after his death, he was revived and then Norman continued to propagate the lie that Harry was dead when he was really just in Europe.
When he came back, weren’t people surprised to find him alive?
No. Months later Peter asks Harry to explain it but nobody else seems to care (Amazing Spider-Man #581… thirty-five issues after Brand New Day began).
Do the ends justify the means?
uh…

Without the deal with Mephisto, Spider-Man would be going through a fairly long and dark chapter in the series. The death of an aunt, constantly on the run, no semblance of a normal life– all while Norman Osborn climbs up the ranks to be the most powerful man in the world. It’s just that Brand New Day maintains that Spider-Man is a fugitive (for being the spider-tracer serial killer and an unregistered super-power) and presents ridiculous scenes, like Spidey walking into a hospital with flowers (Amazing Spider-Man #554).

Give him a tiara and a sash, Dirk. He's the new Miss America!

Epic failure.