It seems to be. Though when I was testing during the most recent betas I wasn't having all data come through. Settings was showing that I had about 700MB of Health+Activity data, but at points Health in iCloud was showing 200MB.
I tested several times by having Health in iCloud enabled, setting up a different phone from a clean start, pairing an apple watch, and checking out health and activity. Not all health and activity data would be visible though. Activity would have rings for most days (but not all), it would have some achievements (not all) and zero workouts.
I tested several times by having Health in iCloud enabled, setting up a different phone from a clean start, pairing an apple watch, and checking out health and activity. Not all health and activity data would be visible though. Activity would have rings for most days (but not all), it would have some achievements (not all) and zero workouts.