Idea Summary
When an Item has Maintain Session State property set to Per User, APEX will save the value in a preferences table. This appears to be read on all requests and the value is added to existing session state. However while inspecting the running application's session state from the developer toolbar, theses values are not displayed.

Use Case
Consider the use of a Page Item in a Server Side Condition. During testing I want to see the value of the Item used in the Condition but since the Maintain Session State setting is Per User, I cannot see the value without going to Administration → Manage Service → Manage Meta Data → Session State → Preferences by User.

Preferred Solution (Optional)
Add User Preference derived session state for page items into the session state views in the developer toolbar.