Watchmen was arguably the best comic book ever made, at least within the megalith of American superhero comics. There was no way the movie could have the same impact on its medium that Watchmen had on comics. The book has its problems, but nothing else in the medium has usurped it from its throne.
Even though I respect some of the creators involved, the Watchmen prequels don't really interest me. I think everything that needed to be said about that story was said in the original book, and I can't think of anything the prequels could add that wouldn't be cliched, pedantic or possibly even diminishing.
That all said, I think the movie was about as good a film adaptation as we could get. And I agree the film ending makes the narrative tighter and cleaner than the book. It still has its problems though:
So cities around the world are suddenly destroyed by power signatures identical to Dr. Manhattan's, who up until this point was known to be the USA's strategic defense system. President Nixon immediately gets on the phone to world leaders saying "we have to band together against this common threat". Well that's great, but notice how every other country lost their capital city, while the US didn't? And conveniently, the city at the heart of what Nixon in the film earlier denounced as the "east-coast liberal establishment", ie, his political opposition.
Isn't the rest of the world, mostly leaderless and in chaos, going to turn around and accuse the US of sacrificing one city full of political opposition to the government in the name of absolutely crippling everyone else? It's not like those kinds of Pyrrhic scorched-earth policies weren't commonly discussed in the Cold War era. At least when you fake an alien invasion, once you establish there is an alien, it's much easier to claim they're a common threat.