BAD THING: This comic winds up invalidating a LOT of what we know about the DC Multiverse's cosmology. I.E., exactly why The Guardians are called The Guardians and one more reason why Earth is so vitally important to the multiverse. I imagine this could be upsetting to the purists who don't appreciate Johns' "Emotional Spectrum" and all the related concepts.
GOOD THING: This. Just... THIS. And the fact that there are still some things in comics that can surprise me.
The Final Verdict: Pure unadulterated awesome for many reasons, though some of the revelations here are somewhat questionable compared to the canon history of the DC Universe as I understand it. Still, if you can get past that, it's a wild read which explains a lot.