I've always felt that once we get a grasp on gravity, the motion of all these objects would be easily reproduced. If your craft relies on the manipulation of gravity for motion, it also cancels out the G effects that are currently our biggest limitation to rapid acceleration. And a spherical shape would be ideal for such a craft, allowing movement in all directions.
I'm curious about this description of a dark cube inside a transparent sphere. Could the sphere in this case be our first visual confirmation of the force that propels these crafts? I've never run across this description before so it's something new to ponder.
If these objects are actually visitors from other worlds, could the different shapes being reported indicate technologies or structures preferred by different species? For example, spherical shapes might seen natural to ocean-dwelling species, but the cubical shape may be more representative of a species similar to humans. It's all pretty fascinating, although I never really followed the UFO folks much (too much conspiracy-theory chat and not really any scientific analysis), so I'm just waiting for the day when they reveal themselves to us, and see what we find out.