Short, spoiler-free version: I have been a fan of the manga since the 90s. This movie was fun to watch, because I already know the characters. If I was new to the series I don't think I would have liked it as much. The characters are much more well-written in the manga. The manga is also more poignant. Overall a worthwhile watch, but it could have been better.
Long, spolier-laden version: I understand how some of the elements from a few graphic novels needed to be Frankenstein-ed together to create an traditional action movie release, and while some of the changes were fine, the chage I had the biggest problem with was the Hugo/Alita love story. It deviated from the original love story 70% of the time, for no reason, really. While it wasn't out of character for Hugo to enjoy rollerball, it was out of character for him to kiss Alita first (the scene in the manga where she kisses him was one of my favorites), he never opted to quit scalping cyborg bodies (he was setup by Zapan for murder anyway), and he would never have given Alita to Vector (Alita game him all the money he needed). Hugo in the manga was a fully fleshed out character with a tragic past and a beautiful balance of childlike hope and adult cynicism. Only about half of the characters were true to the originals.
I don't know why everyone is saying the plot was so true to the original. They must be confusing the word "plot" with the word "setting". The setting was spot on. The plots and subplots were all over the place, unnecessarily. They should have just made the movie from the first two graphic novels instead of trying to cram the rollerball into it so soon into the story. I enjoyedt it because I know the characters, but it could have been much better .