I think I came across a bug regarding the WorkbookContentChanged event. It is fired every time I change a cell, until I use the "drag cell value" feature, and then it seems to be fired only when a new worksheet is created, there are no notifications when a cell is changed (even in newly created worksheets).
PS: I've created a simple example application but I seem unable to upload .7z files. If you still want it, I'll paste the source in a post