When I printed my own designed 'Double Twisted Vase' (http://www.thingiverse.com/thing:37327) I really was impressed by the inside. I thought it would be a great challenge to invert the design so that the outside would have those spikes. And it worked out! I think the result is pretty amazing :) Check it out for yourself! I have printed them with kind of transparent PLA which gives also a cool effect when you put light in it!!

I made two different versions, one with a diameter of 100mm and the other 80mm.

Sliced it with Slic3r and printed it on the Orca. Bottom is 1 mm thick and the wall is two layers =)