I highly recommend it. I never saw the movie but I've heard the differences described as the movie is about the bush administration vs the anti war movement while the book is a very British story about British fascism vs British anarchism. It's not afraid to acknowledge the diversity of victims of fascism and it's not afraid to call fascists losers with weird relationships to sex (sex is a major theme of the book).

Alan Moore is a master of his craft, and his craft is left wing comics. There's a reason a large portion of left wing men are die hards about Watchmen. Vandetta is clearly his first big project, but it hits like a truck and has scenes that stick with you.

It's definitely a book to be aware of political and literary themes in. It's not a story about a noble hero fighting powerful villains. It's a story about someone who's been psychologically broken taking revenge because his fellow victims can't, against weird evil freaks like goebbels, as he tries to help someone who grew up in this world.

Also Moore's forward provided helpful context