I've had the vivoactive 3, 645, Fenix 6, and now the 255. The HR sensor is much improved on the 255 (and similar generation models) compared to older models. It

I've had the vivoactive 3, 645, Fenix 6, and now the 255. The HR sensor is much improved on the 255 (and similar generation models) compared to older models. It obtains accurate HR quickly and doesn't stray or go wild with readings. Enough so that I can go without my chest strap for most runs. I still use a chest strap for high HR interval runs, as it has gone wacky at times here. Another nice thing with the new HR sensor is it uses what seems like a glass cover over the sensor, unlike the plastic resin on older models that had a tendency to develop a spider web of cracks. Another nice thing I've noticed with the 255 is GPS is normally super accurate, and I'm now able to rely on it 100% of the time for auto 1 mile lapping.