It looks great!
The objects seem to stay stable on each other. Normally it is reported as a common issue of ODE that begin to jitter.
The objects look very heavy for the gravity gun, even the cardboard box.

Is there a reason to use strong gravity or a big weight for the objects?
And how does it look with a light weighted object?

http://www.puppenheim.org/
http://www.wuerfels-welt.de
Old Game Tutorial: http://www.puppenheim.org/MITutorial.zip
Free Game Editor: http://puppenheim.org/AlienalsWeihnachtsmann.zip
http://www.puppenheim.org/blender_starting_main_settings.txt