I can mark pills I take as taken using the WebMD app. It's just an extension of what the phone app does. I also have used it for Apple Pay a couple of times after figuring out that you have to actually open that instead of just putting the phone near the reader. The Starbucks card in Passbook works well for payments, but I do wish there was a shortcut. Chipotle looks like it has a pretty quick ordering app that takes your saved orders and lets you place them again in a few seconds all from the watch. Instagram is sorta useful, and the Remote app comes in handy if you leave your phone in another room -- but again, a shortcut to opening it would be nice.
Hey Siri works OK at times. I'm learning to make reminders for certain things on the watch. It also opens Passbook and Remote, maybe other apps that I haven't tried. If a new Apple TV model gets integrated with the watches and Siri, I might mess myself. "Hey Siri, play The Flash season 1 episode 12." Then it starts playing. That would rock.