I'm not a runner, but have found the heart rate to be very consistent and accurate on brisk walks or riding an exercise bike.
The GPS seems to lock quickly, very accurate, and appears to interpolate dropped signals based on steps. I think it's accurate to within +/-2% on distance, and doesn't seem to be biased so it's quite accurate on longer jaunts.
Improved sleep tracking is something else Samsung has promoted, but it's very subpar compared to the Oura ring or Sleep as Android (which will pair with the GW3 to monitor heart rate and motion while sleeping). The other killer feature is SPo2 and sleep apnea monitoring, except the SPo2 sensor has been pretty inconsistent (when it actually works, which is about 1/3 of the time). It currently doesn't track SPo2 while sleeping. It also doesn't track heart rate variability. And I think they dropped the ball not throwing in a temperature sensor - the search for the ultimate wearable continues.
I upgraded from the Huaweii Watch 2 because I was finding it wasn't tracking heart rate very well, GPS was slow to lock and not nearly as accurate. And the GW3 1.4" screen is fantastic. Beyond that, the "killer" health features are pretty much worthless in their present state. The extra storage (which works out to about 2GB extra) was a nice pick-up, and you can download Spotify offline to your watch.