Been playing with the watch more and Siri is one of my biggest frustrations, particularly Hey Siri. It just plain doesn't work most of the time. I'm thinking about shutting this feature off and saving battery as it's basically useless.
Using the crown to activate Siri is better though not perfect. It's pretty slow at times, while pretty good other times. Really annoying to send a text and have to press send as text every time.
Siri on the watch seems to be particularly effected by background noise. In a quiet room it works much better. Even just slight noise causes it to just sit there and hang.