Since I've never read the manga, I can't speak with any authority on characterisation or faithfulness to the source material, but I will say that you can enjoy it without having read it.
I'll give a description of the movie characters and my impressions and you can decide for yourself how much they resemble the manga. Contains massive spoilers, of course.
It opens with a confusing flashback/flash-forward sequence that jumps around a lot. The very first thing you see, other than the notebook being dropped, is a man being killed before he kills someone else as his name is written below. Then, I think, there are more deaths where they seem deserved. These all happen in the opening sequence, pretty quickly. By the time you meet Raito, most of his good deeds have already been done -- in flash-forward.
Once this sequence ends, if I recall correctly, one of the first killings you see him deliberately do is that of the leader of an international group who wants to catch Kira. He's a mere figurehead, but he insults Kira and so Raito writes his name in the book in the street.
(Speaking of which, there're actually a lot of foreign faces in the sequences set in Tokyo, which surprised me. Film representations of Tokyo are rarely so cosmopolitan.)
So, soon after that, I quickly lost sympathy for Raito, although there's always another justified murder to maybe make you think again. That is, until the penultimate sequence, which is fantastic.
Utter sociopath, which wasn't really what I was expecting, since so many people seem to like him. I honestly thought that if he was writing down the names of people that deserved to be killed, I'd probably agree with him, even if the movie didn't want me to. But he was stupid in how he went about it. For a start, he never had any reason to attract attention to himself. He didn't need to taunt anyone. If he's just a student writing people's names in a book, there's zero reason for the police to suspect him. It was his own egotistical desire to be part of the drama that drove him to partially reveal himself to detectives. And if he hadn't done that, then there would've been no need to kill them. Of course, there wouldn't've been a manga or movie either.
(Btw, I got the impression you need to write down the person's name in the original script (e.g. Roman alphabet, kanji, katakana...). Is that so? Just wondering.)
Raito has an amazing voice, I thought. I notice these things.
No, I've no idea how to spell his name, sorry. The first time I saw him in the movie trailer, he terrified me. I'm glad I knew what to expect for his actual appearance.
He's actually a pretty funny character, although sometimes a little difficult to understand because they've put a bass effect on his voice (though nowhere near as bad as computer!L). Best moments included him going mad for apples, walking through a convenience store and telling Raito that he's the Devil. Great line, btw.
Oh, and if we're talking about pairings, Raito x Ryukku. If I knew the manga and could figure out a way to include apples in it, I'd definitely write it!
And while we're on plot points, at some point he tells Raito that he could have shinigami eyes if he gives up half of his life. At least, I think so. Did I mistranslate that...? It never went anywhere in the film, so maybe I misheard.
For what it's worth, even though somehow the CGI doesn't work (I'm not sure why -- the movement are intentionally jerky, he blends into the background well enough and he's nicely detailed), he still scares me a little.
L is love! I fear he might've given kuroe some bad ideas about cake kebabs though. He's cute and vulnerable, likes sugar and is just on the right side of insane (we call it “quirky")... ah, so cute. I liked the scene where he's fallen asleep and one of the detectives puts a blanket over him.
Anyway, about that ending. There's this brilliant sequence where Raito's girlfriend dies tragically and Raito finally shows feeling for another person, screaming and crying and getting in one last kiss before her death. And then it's revealed that... *evil laugh* Really, I'll let you see that sequence for yourselves (although presumably it's in the manga?). It was beautiful.
Then L shows up and he and Raito face each other. L is eating the same brand/flavour of crisps that Raito used earlier in the film. They face each other and...
I'm not kidding. At this point, no one in the cinema moved. We waited until the end, looking for more. And there is. You hear Ryukku's voice and an advert for... the second half.
I've never heard so much talking after a film has ended once the lights went up.
So, how close to the manga is it...? I went to a local book shop today and ALL but a handful of the latest volumes had vanished.
Who's the most right?
If I had the Death Note...
"I could use the Death Note better than Kira."