This engine is fully open-source and have a good community. It's cross platform and you can easily release your game ( and even your app) for iOS, Android, Windows phone. Working and learning the engine is straightforward and it took about one week. the quality of engine (renderer, physics, etc) is awesome and I experienced more than 5 commercial projects. the cocos2d-x version is primly in c++ and It's so convenient to use. It also support scripting as well. recently It also added 3D feature which will make it more popular besides unity3d.

I like the flexibility the framework provides. I've mostly used Cocos2d-x which allows you to create your games using C++, Lua and Javascript. Using the Javascript bindings you can literally create a flappy bird clone in 2 hours or less (assuming you already have your audio and graphic assets). The JS development cycle is fast, lightweight and allows you to test your code in 2 seconds or less (depending on your computer capabilities) If you would like to use Cocos2d Sprite Builder you could also create games easily using the tools bundled with framework.