Horace Dediu tries to figure out how we’ll figure out if the Apple Watch is a success:

1. Language. Measure whether “Watch” will come to mean “Apple Watch”. “Phone” has come to mean not only “smartphone” but also all mobile/cellular phones and not just things used for calling but things used for all manner of information. This is a great test because the theft of semantics can only be accomplished through a degree of ubiquity of influential mindshare. Incidentally, the brand may well have been designed to do just that.
2. A measurable and significant reduction in the use of the iPhone. The Watch peels off uses from the iPhone and therefore the more it peels off, the less remains. However, that which remains will be more uniquely valuable to the incumbent. This is the process of carving and erosion that the PC experienced vs. mobile devices in general.

“The theft of semantics.” I love that phrase.