it's not canonical, it's not part of the original light novel series. The movie is original content. It's different for everyone, i personally enjoyed it but maybe you wont idk. You should also know that there is an alternative/prequel timeline to the main series based on the episode 23 of the main series called Steins;Gate 0: 23β- Divide by Zero and its sequel Steins;Gate 0. You dont have to but i would suggest watching it before the movie