I've been a fan of the manga since I was in high school back in 1995. So this story has been close to my heart for a long time. To finally see it become a movie is amazing. That said, the film is a love letter to the manga. I feel that as far as adaptations go, James, Robert, and Rosa captured the essence of what makes Alita such a wonderful story. Do yourself a favor and see this movie! If you like sci-fi, this film is for you. If you like action, this film is for you. If you like movies with an emotional core, this film has it. Sure it has some "teenage" romance, but it helps to ground the characters in way that is relatable. The dedication from the whole team to bring to life the world Alita inhabits is visible in every shot. It is rich, and filled with a life all of it's own.
Don't listen to the professional critics. This film is worth your time, and will leave you wanting more.