Do you have a smartwatch? Which one and do you use it? What's the most important functionality for you?
Though I must say it is not a must-have, just an extension to the smartphone, which is doing just fine without any other accessories.
The only one main issue with current gen smart watch is the battery life (except the Pebble of course), which can be frustrating because you need to charge it everyday, especially some of them need special charging docks (like Moto 360), but so far I am already used to it, so it was an OK for me, still, if you run out of battery, you can't even tell the time which is slightly not-so-funny sometimes.
Almost all key functionality are completely useless, and Android wear is so poorly designed, that it gets in the way when you actually want to use the small set of useful functions.
The only thing that's relevant, to me, is the notifications. I don't mind the interruptions and usually I discard them pretty quickly on the watch when it's not relevant. In this case, vibrations are way more clever than sound or light, it's a shame that it's the only component that has been left over during the design of the watch :/ (yes, it sounds and feels like an old nokia 3310 on a wooden table)
In a perfect future, you would have a multitude of vibration patterns, sensors and strength instead of a not-working-heart-rate-sensor.