Cartier just announced that the Ballon Bleu is its best selling line. The CEO said it was because in difficult economic times consumers are looking for more discreet timepieces.
I completely disagree with his explanation. I think they're popular because:
1. They look great
2. Celebs are wearing them, giving them more exposure.
Blog here: Cartier Says that the Ballon Bleu is its Best Selling Watch