Now that I have had the vivoactive 3 for a few weeks, I thought that I would chime in with a little detail on how it's been working for me. I have used it as my only timepiece, and I have been rather happy with it so far.
Replacing my Withings Steel was no easy decision, but the more full-featured vivoactive 3 makes the change an easy one. The Withings Steel was great at tracking sleep and steps, but the addition of heart rate and the altimeter really push the vivoactive 3 out in front by a mile. This is probably not even a fair comparison, as the Steel was never really made to be a full featured activity tracker, it is more of a stylish watch with step and sleep counter. A better version of the simpler Garmin vivofit, maybe?