Since the first day with the original Apple Watch. I'd always worn a watch, but I knew I'd never go back to a conventional watch once I had the original Apple Watch for just a day.
It's not that it's a "gadget" to be obsessed with. It just makes things easier for me. I can more easily see if messages, emails and notifications are ignorable or if I need to answer them right away, both for work and personal. A quick glance down is faster than pulling my phone out of my pocket and looking, especially if I'm in the middle of working on something. On top of that, the activity tracking has helped me to improve my overall fitness a bit. And when I'm home, I can drop my phone on the counter and I don't have to hunt it down (or always keep it with me) to answer phone calls, see who texted, etc.
It's not really a separate device, but I think of it like a second screen for my phone -- like a second display for my Mac, it's not required, but it helps me work better/smarter/faster.