iMore wrote a pretty short but very informative article (see the linked article below) about the prospect of leaving your iPhone behind and relying purely on your Series 3 Apple Watch. While this simply won’t work for most people, it will work for people who aren’t already heavy users of their iPhone.
In the next several weeks I will be writing an article on GeekDad.com about how I got my 12 year-old daughter a Series 3 Apple Watch and she is using it without carrying around an iPhone. I personally think this is a great way to go with younger kids for many reasons. The main drawback is that cell carries and Apple still make this very cost prohibitive.