Flutter is a new shiny framework from Google. It started as a toolkit for writing native mobile apps, but Google has been working on Flutter for Web and Flutter for Desktop (they are not production-ready yet, though).
Flutter 1.0 was released on December 4th, 2018 and has been rapidly gaining popularity since then. Although the framework is pretty fresh, there are already multiple mobile apps from big players - like Alibaba, Google and Tencent - working in production.
The development experience is very smooth. We didn't experience any problems while setting up the development environment with a physical device and a simulator for both Android and iOS (on Linux, Windows and macOS). Our IDE was Visual Studio Code which has official plugins for Flutter. Creating a project, detecting a device, running an app, built-in debugger, hot-reload - everything just works. And "assists" that can add padding to widgets, center them or wrap with another widget are very helpful.
The documentation is comprehensive. Besides the usual API reference, it contains a lot of tutorials and development tips, a cookbook, a curated list of sample apps and many more resources. Thanks to that it's super easy to start developing in Flutter.
Another thing worth mentioning is that Fuchsia's (operating system being currently developed by Google, with possiblity of replacing Android) UI and apps are built with Flutter.
Flutter is here to stay. Google is putting a lot of work into it, making the framework a dangerous opponent on the native mobile apps field. Flutter is definitely worth trying.