My Personal Favorite CCK
Sobi was good for a directory type system, even made a website out of it. But, for me, that's where its usefulness ended.
jSeblod, was packed with features but seemed difficult to use. And at the time was not something I wanted to unload on to my clients.
Zoo, was a breeze to use but lacked out-of-the-box flexibility. While the core is free you have to pay for "Apps" to add functionality to Zoo.
Which left K2 and FLEXIcontent. On the surface, once installed, the back-end for both products looked very similar. I even read somewhere that both had some of the same code. But that's about where it ended.
One of the big differences between the 2 was that K2 used its own database tables. FLEXIcontent used the built-in com_content from Joomla. So what's that mean? It means that most (not all) of the plugins for regular Joomla articles will work with FLEXIcontent. And 2, should you decide to uninstall FLEXIcontent then all of your content remains and is still accessible.
Both products require creating templates for displaying your content. One big advantage FLEXIcontent has over K2 is that the templates you create have areas on them. With these areas you can drag and drop the fields you want to use, into any of the areas with ease. Maybe later you add anotheR field or want to moved them around, you just drag and drop the new field into place or move everything around within your template areas. Can't say the same about K2, you have to go in and edit in your changes directly into the template. Not very flexible.
It's a shame K2 has gotten the recognition it has. Especially with several of the big Joomla template makers using it in their themes. IMO FLEXIcontent is a much better product.
Now if can just get a Joomla 2.5 version of FLEXIcontent out.