No such thing as a sure-fire hit, especially in the mobile world. Verizon's Droid looks like the best Android phone to date, but that won't make it a hit in-and-of itself.
The Droid's best feature, over the iPhone, may end up being it's ability to actually make and receive phone calls.
Summary: The phone will be a hit because all the other phones on the Verizon network suck, and because Verizon doesn't offer the iPhone.