As a fan of the manga, it's true that this movie changed a few details, but it manages to catch the atmosphere of the manga. I'm not entirely sure about the actor chosen for Light, although he definitely got better through the movie. I get why he was chosen. L's actor did a great job making me envious of his food XD, and was the one that most resembled the original character. I'm still trying to guess why Misa has black hair now, but I guess they wanted to be more natural looking. Please Misa, give me your recipes :) ! Also, Ryuk looked a bit fake, but who cares about that.
I know I'm funnily describing all the issues, but don't worry, they don't affect the enjoyment of the story. It's a pretty witty movie, it will twist what you see and deceive you all the time. And even when you rewatch it, you'll be amazed at how it all actually happens. Seriously though, who wrote this story has quite a twisted mind XD. I wonder if the second movie (which ends the manga storyline) will be just as good.

Long story short, this movie doesn't disappoint both new and old fans, so do watch it!