Cocos2d for iPhone is a great engine for iDevices like iPhone and iPod Touch.
Not only because its written completely in objective-c but also because its huge class library with a lot of needful things. Starting with basics like render to texture, primitive render and node based scene graph.
You will find support for Angle Codes fonts but also true type fonts. Many already known librarys are supported, like box2d. Audio playback is greatly supported and finally i have to mention that zwoptex is supported (which is a great external tool to create big sprite atlas files).
One big thing is the support of Tiled Map Editor, which is a great tool to create your 2D paralax run'n'jump game.
Cocos 2D is not only a graphics engine since it also supports a server based scoring system to upload and manage your highscores online.
A really big thing in cocos2d is the idea of "Actions" which simply describes what to do with a sprite in the scene. You can chain actions to create complex behaviours and you can reverse actions. There is a huge library of actions to transform a sprite, affect the opacity and color or to animate it via a list of frames.
With Cocos2d iPhone you can quick implement your 2dgame idea or create a highquality endproduct - its up to you.
Cocos2d is small - so it will be easier to hold your Cocos2D-App under 20MB. Why is that so important? In some countries telecommication carriers provide a limit of 20MB for App downloads on the fly. Apps over 20MB then have to be downloaded via WiFi (or via iTunes and then synchronize..bad) - this is a big issue for marketing. The count of downloads will rapidly increase if your app size will be under 20MB, trust me!
Although this is a great 2D engine, 3D elements still missing. I see forward that once there will come a time to advance it to a cocos 3D engine. But many projects show that you dont need 3D to create a great game.
This engine is upgrown (although its still growing). I would recommend this engine to everyone who want to start iPhone Development and also for those who want to create a highquality 2D game for iPhone or iPad. Cocos2D is easy to learn, fast, uses OpenGL, has a huge library, supports many useful tools, has a great community and last but not least it will not break any TOS Apple ever releases!