It support Lua and Javascript with full feature support. Especially with Cocos2d-JS you can develop games cross web and native, and the native solution have great performance with JS Bindings, much better than hybrid solution

According to the Cocos2D-x website, 25% of iPhone games are made using Cocos2D-x (A Cocos2D mobile variant.)

This means you will not be alone in development, and will have access to a large community and know you are developing for an engine that works.

Cocos2d-x forums are active.

Cocos2D-x is not only open source but also supported by Chukong Technologies of China and USA.

Regularly updated and adding support for the latest technologies. 2014 has already seen the release of Version 3, a new Cocos Studio development toolkit (optional) and support for new technologies like skeleton animation systems Spine and Adobe DragonBone.

Documentation includes a programming guide, API reference, video tutorials and massive reference test code project showing all functions and giving working code to the user.