Watched the movie, then read the book. The movie does a pretty good job of carrying the main plot over. They changed the manufactured alien to a nuke, but really it is the same outcome, and frankly, the nuke(Dr. Manhattan) is a way better device.
Although, I think the nuke misses the point of the ending, where it seemed like the cold war was only prevented for a time by the alien. Like it seemed the news was reporting how everyone was becoming increasingly skeptical of it, and it ends up not detering the war from occuring at all. Which is kinda what Manhattan says to Ozy at the end.
And I haven't even seen the best director's cut (although I saw some of the added scenes). I imagine the director's or ultimate cut or whatever, is pretty darn good as an adaptation.
A series would work well, but is kinda redundant at this point. Maybe in a decade or so.