I've seen them before, and in the hands of agood shooter, they seem to be very effective. I think they go something like 2.5" dia. for the 100 gr. heads and 4" in the 125gr.
I would think that since the blades are rather thin and perpendicular to the line of flight they would tend to break very easy.
I've seen them in stores and they go for about $12 a head, seems a bit pricey since it'll only work with turkey.
http://www.arrow-dynamic-solutions.com/gobbler_guillotine.htm#THE%20GOBBLER%20GUILLOTINE