I thought the basic idea was completely derivative of other, similar sci-fi "change one thing in the past" stories.
I also didn't know that Barry Allen's mother being murdered was a part of history? When the hell was that part of his character. What a TERRIBLE idea!
How many of these characters now how have a parent dying in front of their eyes? Batman certainly, Hal Jordan, Superman (w/ Man of Steel movie) and now Barry? No no no no.
It totally changes these characters. And in my opinion it RUINS Batman. Barry doesn't have to be about "Hope" by having the same damn origin as Batman but not being revenge driven. I think it's stupid.
Also there are a couple of things that don't quite make sense. For example, why is Superman their last great hope, when the Shazam kids are technically just as powerful?
However, having said that, what was fun about this storyline were the interesting alternate takes on characters like Aquaman and Wonder Woman (who apparently should never sleep together... even though it would make sense for them to be together... all mythical and whatnot). So I enjoyed it.