Hi,
For my first Camino (summer, 2007) I purchased a shell in Puente la Reina. However, when we arrived in Fisterra one of my companions collected a beautiful small shell for each of us as a gift. For my second Camino (summer 2008) I drilled two small holes in it and sewed it on my pack. It wasn't as big or jangly,(is that a word? or am I making it up?) and it didn't have the Santiago cross painted on it, but it had special meaning and I was very pleased to take it along with me.
I have found that when I am in other places in Spain and see people with a backpack, if I note the vieira I will sometimes speak to them about the Camino...at least you can identify them as Pilgrims not just backpackers or hikers.
Buen Camino,