I've got this jacket armour and it's pretty damn good. I actually don't think there's any hard shell armour on the spine, only approx 10mm padding. I've tumbled onto my back lots with it and it feels great, like Lonerider said nothing will save your back if you taco on a rail or knuckle! lol
I ride quite aggressively, and sometimes take some nasty falls, and so far the armour has saved me from getting seriously hurt more than a few times.
I have a friend who bought the whole setup from skeletools (sp)?
I tried it on at his house, makes me look like a Superhero.
That said, If I ever get the courage to start doing tricks on rails again I'm going to rock it.