In reply to Nik:
There is an interesting HSE report looking at the safety of ascenders, including the shunt. It was written in 2001 (by Lyon Equipment) so doesn't include the Duck. Google CRR 364/2001 if this is your sort of thing.
The report suggests that the factors of safety of the shunt are low, with the frame bending at quite low (static) loads. As Adam above says the Duck is an improved design in this respect.
Having said this I still use the shunt, a fall during self top roping should only be a sag onto the rope, which generates very low forces. If the shunt slips, which has never happened to me, and slides until it hits my safety knot, I think the impact forces should still be tolerable, although these forces will be lower with a dynamic rope. The dynamic tests in the report used a fall factor of 2 so very severe and certainly not what would be experienced during top rope soloing. So as long as you don't grab the rope or shunt itself during a fall I think it is a reasonably safe device.