I use mine for both running and weights. When doing my strength training routines, I set the workout app to Other. The first time I selected that, the watch let me know that if no other sensors provided data (i.e. HR), it would at least be tracked as the equivalent of a brisk walk. HR sensor works fine for me when lifting, and I see it rise up and down with intensity.
My strength training is a personal hybrid of HIIT and BFL. It takes me all of 12-15 minutes and I get the results I want... not a body-builder, but lean and defined. I alternate between upper and lower body groups each session, and alternate between strength training and cardio daily with Sundays off. Everyone has to find what works for their body.
I am very impressed with the HR sensor on the Apple Watch vs. my Moto 360. The AW is able to continually track HR even when I run, while the M360 just gives up with an error message. It is nice the AW can pair with a chest strap, but I'm happy with it as it is.
Now if they can add Oxygen Saturation via a software update, that will be very cool.
One last note: I was playing around with VimoFit on my Moto 360. It can track lift reps and sets via the gyro/accelerometer. Their web site said they were working on a version for Apple Watch. That would be neat. Right now I use GymBook on my iPhone to track my lifting sessions. No AW interface yet.