Back to MAC

Has anyone ever done the back to MAC program? I have to admit I never knew you could do this until about a week ago. I was wondering why all the boxes say back to MAC on them, so I googled it and found a page on MACs website which says that if you keep 6 empty containers and take them back to a MAC store (you can post them back in the US but not here) you can choose any free lipstick (but not limited editions like Viva Glam). Here is more in depth info - http://www.maccosmetics.co.uk/customer_service/services.tmpl

It's a pretty good incentive. I have one empty so far :)