I think the general consensus will be that the battery life is WAY below what we need it to be. But whatever the battery life does end up being, you'll have to charge it every day for sure. The issue as I see it will be how much 'screen on' time Apple has accounted for in that day.
At first, people will be checking their watch more than their phones. It's a new shiny toy which has to be played with. That will result in reports of watches dying by 11am, raising questions of "how should we use this thing" etc.
It's such a new industry and nobody really has any idea where this is all going. But as long as it looks great, plays nice with iPhones and has great apps I think it will do very well.
Expect 2.5 hours of heavy use, 3.5 hours of standard use or 2-3 standby (i.e. doing nothing) time.
I can kill my iPhone 6 by 11am. Battery tech is shit.
Talking of which, has anyone been following this almost unreal-sounding charger: https://www.kickstarter.com/projects/265641170/kraftwerk-highly-innovative-portable-power-plant