Also, cool demo video here (yeah it's an ad, but the demo looks legit):
https://www.youtube.com/watch?v=hWl2Hp-sU2c#t=66
It actually is really scary how easy it is to kick a door in, there's really not much protecting you at all, literally about 1/4" of wood. That's it. Typically doors open towards the inside too. If it opened towards the outside it would be harder to kick in, but then the hinges would be accessible. Can't win.
This system is a neat concept, though they should look at making it automated, like a button you press and it just slides up or down.
But bottom line is the windows are another big weak point so if the door is too hard they'll just smash a window.
Best bet is an alarm system for when you're not home, and a weapon for when you are.