I've got CarPlay in my new car; and in my old car, I used the AW with the iPhone -- but without CarPlay.
I don't really pay attention to the Watch in the new CarPlay-equipped car as much as I did in the old car. The dash display is plenty big, and it's easy to read and tap the controls.
The AW was great to use without CarPlay, because its taps and "ding-dong" chimes for navigation came in on time, and it showed the next turn quite clearly. But it's a little redundant with CarPlay.
It's caused no problems, though. The AW still receives notifications and nav directions from the phone while I'm using CarPlay. Maybe it'll work for music controls and texts, but it's simply easier to use the dash display and steering wheel controls.
I also suppose that I could plug the charging puck into one of my car's USB outlets and top up the AW's battery on a long road trip if it ever gets to that point.