For sure, anytime something hits legend status, you should always be very cautious at later works like prequels and sequels and whatnot. Watchmen is at the top of that list as far a comics go. I've been highly impressed at the way Johns has done it. Nothing feels cheap or like blasphemy. He's done all the characters justice. I don't see where he's tarnished anything from the original. Just a really good job by him. I'd say it has the potential to be an instant classic.
I think some are going to have problems with the ending, I just don't think all these universal rebirth continuity questions will get answered and wrapped up nicely. I'm just taking it as a standalone, kinda sequel that touches all of DC.