I have a few high end fitness watches. A Garmin fenix3, Polar V800, and Garmin vivoactive. I have also owned a fitbit Surge, and Charge HR.
Although they may be more advanced for fitness, I like the simplicity of the Apple Watch better, and it seems to be much more accurate as well. The issue that someone mentioned about counting reps while lifting weights is shared by all devices that read your heart rate optically. The muscles constrict the blood flow and throw the optical reading off. The apple watch can sync with blue tooth heart rate monitors for those types of workouts, so that would eliminate that problem.
I power walk every day and compared the heart rate of the apple watch with my Polar V800 chest strap heart rate monitor during a walk, and both were within 1 beat of each other.
The thing that sets the apple watch apart is that you have to do 3 things to make your daily goal instead of just getting so many steps in. You have to meet your calorie goal, your exercise goal, and your stand goal to meet your total daily goal. That IMO is better fitness tracking than just how many steps you take in a day.
There are also a few fitness apps that will work with it, and I am sure a lot more to come.
[doublepost=1453612888][/doublepost]
If you do the calibration with the apple watch it is extremely accurate. Search Consumer Reports for the testing they did on the apple watch compared to several fitness trackers.