Ranked in these QuestionsQuestion Ranking
Pro Plugins for popular IDEs
Telerik provides Visual Studio and Sublime Text plugins for the AppBuilder.
Pro Integrates with Apache Cordova
AppBuilder offers a fast way to import Cordova plugins such as the camera.
Pro Great developer tools debug/hotreload/analyser
Pro Based on Dart language
Dart is a Java like language, easy to learn and startup fast for millions of Java developers.
Pro Open source
Pro A single codebase for iOS AND Android
The developer's goal is to allow people to make apps running at 120 FPS.
Con Young project with a small community
AppBuilder is a young project that's been fairly slow to gain popularity. This means the community is rather small, so there is a limited amount of 3rd party learning resources available.
There is a forum, but it's not very active.
Con Still in development
According to the website, Flutter is still in its early stages of development.
Con Based on Dart language
Dart is a Java like language, easy to learn and startup fast for millions of Java developers. BUT if you have to learn it ... it's a con.
Con Dart is unpopular and never gained serious community traction like Kotlin or Java
Con Does not support 32-bit iOS devices
If you plan on targeting iPhone 5, 5C or earlier, you can forget about Flutter.