Its really great.

This has long been the best app for recording workouts using a Bluetooth HRM, and now you can also use it with an Apple Watch, both as an app and a HRM. It is extremely customizable while remaining pretty easy to use. The charts and comparison tools are great for analyzing your progress as you practice. I use it to record all my kettlebell workouts with my Bluetooth HRM and I am going to start using the Watch app to record my active recovery workouts, at which point I will be using it almost every day.

BeepGirl about Heart Graph, v4.0