I've been running my watch alarm slightly offset from my phone alarm to see if it will work. So far, it wakes me up just fine, and I don't even have it on the loudest setting.
I think it works well right now partly because I'm not really sure what to do with it when it's going off. With my phone I have a bad habit of picking it up, unlocking it to disable the alarm, then browsing Twitter for a few seconds and falling back asleep.
With the watch it's like... I look and wonder... do I swipe? No. Sleep calculus intensifies... press the crown and tap to dismiss the alarm and then what? I guess I should put it back on the charger? I dunno. By this point I'm usually awake enough that I get up and do what I need to do at whatever time.
So, I guess I won't know until I get used to it if it will work in the long term, but, it's waking me up for now.