Since my current wristwatch looks like this:
The size and shape of an Apple Watch is a complete non-issue to me. The battery life only really needs to be 18hrs too since I can charge it every night no problem.
For me, I can see use scenarios around controlling my ATV and AirPlay music from the watch. I don't always have my phone in my pocket in the house.
Genuinely silent notifications (vibrate on the phone is really quite loud) will be useful in work. I will turn vibrate off on the phone and may even turn the ringer off all the time, too.
In my car I have a Bluetooth dongle which means I can play my music through the stereo but can't use Siri via the car phone mic. I can see scenarios where using Siri via the watch will allow me to voice-control my music, even if my phone is in my jeans pocket (very hard to get out when sitting in the car) or in the boot (trunk for the colonials).
I am planning to buy one for my wife, too, as she never hears her phone ring when it's in her hand bag. This drives me nuts!
And I like gadgets.
Like it pigoo3 said on the iPad, I'm sure there will be other usage scenarios too which I can't anticipate yet. I would find being without the iPad a major inconvenience now.