While relatively few Watch-capable iPhone owners are getting one, current sales are not bad at about 5% of iPhone buyers. Very similar to Apple Pay adoption.
(Although it's not like iPhone owners have any real choice when it comes to a fully compatible watch... Apple has made sure of that by doing what Microsoft used to do: keep the tightest integration APIs to themselves.)
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The bigger question to me is: how many end up in a drawer somewhere? Historically speaking, about half of smartwatches end up not being used after about six months. People forget to charge them, bring a charger, etc, and eventually fall out of the habit.
I'd love to see a survey to find out if Apple has beat the normal odds on long term user adoption.