Useful workout functions include an optical sensor for heart rate measurement, along with bearing, altitude/barometric pressure, and temperature sensors, and an accelerometer for step counting.
These five sensors help to keep you in close touch of your current activity in real time. The ability to receive GPS signals lets you access location information when you need it. This, when used in combination with the stopwatch, makes it possible for you to keep track of running information like distance, speed, pace, and more. Your heart rate and speed is used to calculate a VO2 max value, which is an indicator of your cardio-pulmonary capacity… More here.
I always felt that Casio was well placed to make a true smartwatch and this looks pretty good. It is still more than a bit chunky though.