I actually think the watch will have huge impact on heart disease (and am working on making that happen). The resolution of data you can get on the watch is just amazing, but we'll need some basic research to translate that into real medical gains. I'd expect to see impact in the next year or two though.
I've been wondering how good the continuous readings were on it. I'm actively considering the best device to track my really complex heart rate issues (I have postural tachycardia).
But I don't use an iPhone, so Watch might be off the table for the near future.
My personal experience is that the Apple Watch's heart rate readings are usually within 5 minutes of my AliveCor. Unfortunately, none of the Android watches currently on the market have a sensor that's particularly reliable. For now, I'd say a Garmin device or an AliveCor are the best bet for non-iPhone users.
How does the watch compare to other heart rate monitors on price, reliability, communications, etc?
If the watch can learn to detect irregular pulse/atrial fibrillation (hell, even if it only recognizes it a fraction of the time) it's going to have a huge impact on strokes, too.