It will sell because its apple. Any shortcomings like the price and battery life will be overlooked by the tech press and users and it will be proclaimed the "best" wearable in their blind rush to praise anything with the apple logo on it.
Not to all Tech press, and that is what I watch for. You have your Apple fanboys who do press, and of course they will claim "best thing in the world" but, as you get to "real" and "honest" reviewers, they will give the honest thing on how it is, I am not going to sit here and say it's a horrible OS on a watch, chances are it's pretty good, anything can be better than the iPhone.
The battery life thing will be a big issue, I am sure for the hipsters you will see them sell a lot but, in the long term if battery life is really poor and cant be addressed via software updates, It will be a failure for Apple.
The Smartwatch is still a specialty thing for people. If they are doing fitness, a $139 fitbit would do everything they want and work with MOST (exclude WP for the most part) phones. If they want something for business, yea, it might work but, if they battery life is really bad, you will find these people selling their iWatch for something with better life.
This will sell for the specialty market and the techy people but, mainstream like a iPhone? never going to happen, at least for the next few years...
The really sad part of these, even for Apple here, we are using new tech, better CPUS, better displays, better chipsets (for features on the device) but, we are still using battery tech from 10 years ago.
I'll be impressed when they can make a device like a phone, last for over a week with HEAVY usage and is smaller than a penny... 1/2 that size in a watch, could last a month....then we are talking