Personally for side scrolling games such as 2D platformers I like to mix a traditional tile map system with hand placed graphics and objects. Using editors such as Tiled and tIDE you can create levels very quickly. By adding a layer of hand placed graphics and objects over the tilemap you can make a map much more interesting and dynamic than you can just with a tilemap alone.
I use tIDE personally and really like it. However, I don't know how easy it would be to integrate with cocos2d since I've never explored its import/export functionality. Tiled is very mature and a quick Google search finds many tutorials to help get you started.
I have an in game editor to hand place objects that are not tiles. However, you could use either Tiled's or tIDE's objects layer and properties to align things for you.