It is a big task but I would really love to have multi selection work. You can check Unity for reference, they do a great job with it.
Selecting multiple types would only show an intersection of properties in the properties panel. For all properties that have different values you would just show [Multiple Values] inside the text box or in case of a checkbox it would be the one with a [-] inside, etc. but then when click on it or setting a value it would set the same value to all of the selected objects

I would say that it is very useful if you improve on Unity way of doing it, because Unity for numbers when you drag over the textbox they just move all number to the same position and it changes the multiple values text to the actual number (setting same number to every selected element). I think in this case it should just increase/decrease the values the selected elements have