I tried a fitbit when it first came out due to curiosity, but ended up not using it that much. I am an active individual and didn't see how it helped me. I know a lot of people liked them initially, but I saw it more as a fad that would die off or become integrated as a bullet point feature in future products, especially for those that don't use them as actual training tools like marathon runners.
I think the Apple Watch could be something big a long time from now. I think it needs to be more worry-free such as kinetically/solar charged, integrated into many facets of life, and ubiquitous commercial support. The day when my car is programmed into my wearable and walking up to it unlocks the doors, sitting in it starts the engine, approaching my house while in the car opens the garage door and unlocks the front door, etc then it will be a nice item to have.
I think it can get there, but right now its a tough sell. It is a mega investment by Apple to try to get to that point and hope everyone jumps on board. However, its not only a big investment from Apple its also a huge investment from the consumer and third parties. It still unsure automation is worth it to the mainstream consumer on such a scale where it does truly become ubiquitous.
In the end, the wearable could displace phones.