According to theAppleBlog, sales of SL are double that of Leopard.
Most will agree that the price is pushing sales high, but I also think Leopard was just not good enough. At least in my case, and talking to other Apple users, a lot of them aren't (weren't) really happy with Leopard. Leopard was suffering from "Windows syndrome". Being bloated and slow. Some of it due to carrying legacy PowerPC support, some of it by feature creep and unoptimized code. In SL there are still some loops open, but that is mostly in the application area. (iTunes is still carbon forinstance). That will change in due time (I predict an "iMovie08" makeover for iTunes next year) The OS is finally mature :-)