So far it has been even more stable than iOS 10. And iOS 10 has been extremely stable. One of the best beta's ever introduced. I have also been running the developer beta on my watch since day one. Not a single glitch and some really nice face customization being added to watch 3. Some new features in Messages as well.
There is a real risk in running it though. This is not to scare you away from it, as I am running it without any issues. Should you be one of the unfortunates that has problems with it, the only way to go back to the previous OS is by sending your watch into Apple to have it brought back.