At the two week mark, OS X Snow Leopard—despite having few user recognizable features—has sold more than twice the number of copies that Leopard did, and four times the number of copies that Tiger did. That makes sense given the $29 price and Apple’s higher percentage in the market these days.
Surely the $29 price — a full $100 less than most OS X upgrades — is helping drive sales, but I also think users see Snow Leopard as less dangerous of an upgrade.