The problem with all the third-party apps at this point is that they do not have access to the HRM of the Watch. For me, that's a huge downside, as I want get as accurate of a calorie burn calculation as possible.
I have one with my Garmin, but I never wear it. That was one of the selling points (for me) of the Apple Watch.You could use a BT HR chest strap and Nike can read from it.