I can explain it.... it's a real pain in the a**! No design-time support, and it has some odd behavior to it. For instance, in collapsed state there is a large margin at the bottom of the control (you'll need to handle the collapse event and change the size of the control to 25 I or something like that).
The way I've worked with it is to create usercontrols that have all of my treeviews, buttons, etc, and then add the usercontrol to the panel programatically. You need to position and dock it yourself through code based on the panel's current height, width, etc. Also, you'll need to expose events and properties to your usercontrols so that they can communicate with each other.
I've since created my own version, going so far as to duplicate the look and feel of the collapsible panels that Autodesk uses in their palettes. Works very nicely and has design-time, drag-and-drop ease of use. I'm going to add support for embedded buttons in the panel header that can be turned on/off and the ability to set the button images. I'm also working on making a custom layout container (or several) that automatically enables collapsing behavior and sizes each panel. Perhaps when I've added a little polish I'll make it available... (for VS2005 only).
