I love K2, and Ive played with a lot of other CCK solutions, some are great but often its more than you need when K2 is almost perfect and you dont want to start from scratch building a whole app through another CCK. 

In this case, I needed to customize (read: simplify) the backend of K2 for a client and I realized it would be so great if I could control the backend template per category just like I can for the frontend. So I put together this hack. Its not perfect, but it works.

This way, I can have some categories whose admin pages show their extra fields in the first tab, or perhaps just the text editor and video settings, depending on what I need for that content, without confusing the client with lots of options they dont need.

In theory, this setup combined with extra fields and the ability to call specific extra field data in frontside K2 templates basically makes K2 as versatile as any CCK, without the need to build it all up from scratch. For most projects, K2 is a perfect base, and with these two additions I can customize it more than I ever might need. Of course, either of these might be useful to you on its own as well.

Check out the code and explanation here: jurawa.com/notes/item/27-making-k2-more-like-a-true-cck-custom-admin-templates-and-calling-specific-extra-field-data

The next step will be to figure out how to make all this into a plugin, but im far from that at the moment...

Also, I havent tried it yet, but combined with the fields for k2 extension this could be pretty sweet.