I would say the best part about it is the hot reload, which maintains application state. It's compiled to native code so it's going to run faster than ReactNative which is a javascript bundle that runs besides your app. Compile times are fast, but not instantaneous and you have to trigger the reload manually.

I think it also has the most complete implementation of Material UI out there. It implements widgets that are not implemented in iOS, Android nor Web. It's like having something like bootstrap that makes it easier to build a good looking app. I feel like you really need a mock first to build a reactnative app, but you could get away with just writing code in flutter.

Unlike ReactNative there is no way of trying it without installing the Android or iOS development environment. Initially that was a bit of a barrier for me as a first-time mobile dev, but not a big deal eventually.

It's not as mature as ReactNative. There was no build-in way to do audio, something I need for the app I am trying to build. However, I found a plugin for it on github.

I know Google has a reputation of canceling projects, but Flutter is used to develop the Google AdWords app so I don't think it will go away anytime soon.