Is this purely a technique thing or will it have big effects on the body too? Is it a particularly good cardio exercise?
I think the most useful thing about shadow boxing is technique, enhancing your muscle memory. Throw punches in correct form, practise footwork, make sure your stance is correct, etc. Most people shadow box but they don't imagine an opponent in front of them, shadow box as if you're fighting. Feint, move laterally, throw combinations, slip, duck, etc.
Shadow boxing is a good way of practising drills without a training partner. You should be concentrating on choreographed sequences that you would use against an opponent. The advantage of shadow boxing is that you can do it any time of the day or night.