I think that they're a fad in the same way that 1990s smartphones were a fad: the technology to build good ones doesn't exist yet. A watch needs to have a battery that lasts long enough that I never accidentally forget to charge it and end up with it not working (my current one is on its second battery and the first one lasted about 5 years) and be light enough that I don't notice that I'm wearing it. I have both of those from a Skagen watch, but if I could keep those requirements then I'd find it very useful to have things like my day's calendar sync'd to the watch, to be able to use it with Bluetooth for two-factor authentication, to be able to use something like Apple Pay and leave my wallet at home, and so on. Make it a quarter the current thickness and make the battery last a week and I'll happily buy one, but that isn't possible yet.
The same thing was true of Smartphones. It was obvious before the iPhone that there were a lot of useful things that a Smartphone could do, but until LiIon batteries, low-power WiFi chipsets and screens improved to a certain point, the downsides outweight the benefits. The difference between the iPhone and the Apple Watch is that the iPhone was released at precisely the time when the technology made it possible to build the useful thing, whereas the Apple Watch appears to be 5-10 years too early.