Ranked in these QuestionsQuestion Ranking
Pro A proven engine for mobile development
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. You'll know you are developing for an engine that works.
Pro Supports 3D models with skeleton animation
A new feature since Cocos2d-x v3.1 is support for 3D models (in your 2D game), not only this but support for skeleton animations is included too! This awesome feature allows for impressive characters in your game along with easier, more fluid and realistic animations.
Pro Great script language 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.
Pro Allows for easy debugging
It has a built-in Python interpreter that allows for easy debugging.
Pro Great video tutorials
Pro OpenGL hardware acceleration
Pro Highly active community for questions and support
Pro No external dependencies
Because it is based on Pyglet.
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.
Pro Greater performance than high level APIs
Cocos2d-x is C++ based engine and it has CPU advantages for most platforms because of that. It uses polygonal mesh methods for sprite rendering for using GPU advantages. (You also use quad methods for benefit CPU).
Pro Easy integration of 3rd party plugins
For example, if you want to add a rating plugin, you use
sdkbox::PluginReview::init(); and if you want to add the Vundle Ad Network SDK, you use the one packaged in SDKBox
Con Poor documentation
Most existing documentation is out of date. API documentation sometimes exists, though is usually just a one-liner and more often than not is completely absent. New features and significant system rewrites often make it to a major release without a single line of documentation accompanying them. Cocos2d-x family is developed by Chinese mostly, so market outside of China is not their main priority. Some tools (like Cocos Studio) have China-specific services and markets.
Con Cocos2d-html5 is way behind Cocos2d-x
While Cocos2d-html5 tries to mimic it's Cocos2d-x sister and has the same people working on it, it has lots of restrictions compared to Cocos2d-x: no Spine mesh support, no 3D models, bad text label quality, etc.
Con Poor support and non-existent community
Up until 2013 this was one of the best engines around. However, since then it was bought by a Chinese company and began stagnating - it's virtually in slow death. Most developers abandoned Cocos in favor of more modern solutions leaving the community weak and the forums with little or no traffic. Although, the Cocos2d-x Forum seems to have a decent community going.
Con Fragmentation; hard to find useful tutorials
Cocos2d family includes different projects: cocos2d-x, cocos2d-html5, cocos2d-objc, cocos2d-XNA. Most tutorials describe techniques specific to single project of the family, and cannot be applied to other projects.