Coincidence that the only other live-action adaption of a manga that I've watched is Solanin?
Overall this adaption is well done due to two main reasons:
1. The acting brought out each character's charm, especially Koyuki's awkwardness and Chiba's impulsiveness.
2. Good choice of what was modified from the original story and left out...
...except I wish they spent more time showing how special Eddie is to Ryuusuke. They could've easily included the explanation for why Ryuusuke got so mad at Koyuki when he broke the guitar lent to him (it was Eddie's). They could have also included the subplot about how Leon Sykes killed Erica, and her link to Eddie. That would've also added to the fearsome image of Sykes, which I felt was inadequet. Ran also wasn't scary enough relative to how much of an obstacle he was in the manga. Lastly, I can't believe that Koyuki's rebuttle to Yoshito re: Maho at Greatful Sounds wasn't included, it's such an important indication of his growth.
I grudgingly accept that not including Koyuki's singing voice is a good decision. It was a huge disappointment since my main motivation to watch the movie was to hear him sing, but I would've felt even more disappointed if his voice wasn't as magnificent as it should be. Given how high the standards are set, it's a practical decision to leave it out.
However, I can't accept that the soundtrack doesn't include any of the actual songs from the movie. What the actual fuck? But I'm appeased by the choice of Oasis for the ED song, although "Stand By Me" would've been more appropriate.
In the end, there's no way a 2.5hr movie can do justice for a 34 volume manga. So definitely read the manga first, and watch the movie for nostalgia and how cool Koyuki looks when he sings.