I don't know if it's the major contender for React Native, but it's definitely a competitor.
Personally, I love it. It's declarative UI syntax is really easy to use, and you don't have to worry a lot about constraints.
However, coming from an HTML/CSS/JS background, React Native was extremely easy to pick up, and I had fun working with it.
I will say that Flutter's system of running the app seems much faster than React Native's, especially with the live update.