An adaptation like this begs for a metaphor. My first reaction was to make it musical: that this movie has the same melody line as the graphic novel, but is missing the harmonies. That's not really right, though: it has its own harmonies -- perhaps less complex than the original, but there nonetheless. Closer to the mark would be to say that these are two tunes built on top of the same tenor line, but that's not really right, either.
So a different approach, which I suppose applies to most good comic book adaptations: this tells much the same myth as the original story. But as is the way of most myth, it has changed in many ways, some of them important. It is trying to tell a different story through the same myth -- thematically close, but not the same.
That said, this is a good movie, and I don't really begrudge them their changes. For the most part, they understood which scenes in the original were most powerful, and largely didn't mess with them. In particular, the critical chapter of the story -- the transformative sequence that I regard as the best single issue in the history of comics -- is preserved almost word-for-word, and they do it justice. As I expected, they greatly compressed the story. They sort of had to: the original is both long and intricate, too much so for a 2-hour film. But they did so with care, and the result is a movie that is quite powerful in its own way.
There are some missed notes. They make a number of subtle changes that have the result of humanizing V much more than he is in the original. That's not an unreasonable decision on principle, but there are times that he comes across as -- well, kind of a lovable goofball in a crazy way. IMO, that's taking it a bit too far, losing some of the intimidating power of the character.
Perhaps more seriously, I think the movie both oversimplifies and oversells the political message, which comes across as a bit ham-handed. The bad guys are more simplistically evil here, villains in search of power. What makes the original so unsettling is that the evil in it is so banal. In the movie, you can almost dismiss the villains, saying that real people wouldn't be so bad. But in the original story, the fascists (I don't think it really gives anything away to say that this tale is a screed against fascism) mostly sincerely believe that they are doing what is absolutely necessary. The terrifying thing about the original is that its scenario is not just plausible, it's almost inevitable given the history it presumes.
Still and all, by the numbers this is a good movie. The writing is good if imperfect, the direction crisp, the acting excellent. Natalie Portman is cast as the heart and soul of the story, and she carries the part ably. For a Wachovski Brothers movie, it is surprisingly restrained in the action sequences and special effects -- they're there, but they don't overwhelm the story. (They only succumb to bullet-time sorts of effects in one or two places.) And while the political message is a bit unsubtle, it's a very timely message to tell.
So the movie is recommended. (Although let's be clear: this is absolutely not a film for kids -- it is very intentionally violent, and that violence is central to the story.) But it's no substitute for the original graphic novel, which is longer, subtler and rather darker. If you like the movie, I do commend seeking out the original, which is currently easy to find...