I just picked up the Alpinestar Tech back protector from Road Rider the other week. Very nice construction, seems like it is built to last many seasons. My favorite is the cross strap design, you can easily tighten while having your jacket on.
When I first wore the protector I wasn't sure if my jacket will be too tight, it's not. After a few rides the back protector stays in the correct area, does not move. Also it's pretty light, with the padding your back does not notice the protector at all, and I was wearing a back pack on top of my jacket.
Paid $125 for the medium size, feel it's worth the protection for your back and it's a quality item.
Rob