I haven't owned any of the current generation of cellphone-accessory smartwatches. The ones I have owned:
- -- Casio GPS watch - It was a gift from my wife, back before GPSs had taken over the world. It was big and clunky, got me all kinds of geek cred at work, didn't work very well as a GPS but the fact that it worked at all was amazing.
- -- TI EZ430-Chronos watch - programmable, using their MSP430 microprocessor set, had a reasonably flexible display. It didn't have a lot of sensors, and I didn't end up hacking it very much, but it was a lot of fun. It had a low-power radio link that let it connect to a heartbeat monitor band, so you could use it for things like watching your heart rate while jogging.
- -- Watches with various other functions built in, like moon phase, tides tables for surfing, that kind of thing. One of them had a screen saver for entirely no good reason, just because it could.
In practice, I find that almost all of the time I'm either in front of a computer screen with a clock display in the corner, or in an environment with clocks around, or carrying a cellphone with a clock display on the main screen, or in an environment that's not very friendly to watches, or in a social environment where I don't really care what time the clock says it is, so I've stopped wearing watches most of the time.
When smart-watches get smart enough to be the phone instead of being a peripheral display for the phone, maybe. But is a smart-watch phone that needs a Bluetooth headset and needs reading glasses to use more convenient than a cellphone with big text that can use a wired headset? For me, it's really not.