People who want a fully independent Watch will probably be disappointed, but for the use-case of "I want to leave my iPhone at home while I go out for a jog", the fact that the watch uses the same phone number as your iPhone is very good indeed -- having yet another number to manage for the relatively short duration of a jog/swim seems ugly.

So it has a software SIM? If so it's the first time I've heard of a device with one.