Tracking when I workout, notifications (I carry two phones and during the day my personal phone is in my bag most of the time). And just using it as a watch. I hated that most of the "smart watches" out there would either run out of battery in 8 hours, or have the screen off by default (with a wrist action or a button to enable it).
I don't use the sleep tracking at all, hate having a watch on at night, but the step tracker is fairly accurate and useful.
It's one of those things where I initially bought it for the notifications and the (supposed) battery life. And every now and again I'll find some random new cool use.
I've always had nice wristwatches (and prefer to look at my wrist for the time instead of my phone) but I've been thinking about buying one recently. Most of them seem centered around health / fitness tracking, though, and that's not something I need or want. Mostly, I just want a "regular watch" (date/time) and perhaps notifications of incoming calls/messages (not necessarily even the content of messages -- just, for example, "SMS from Alice" or "Call from Bob", enough to let me decide whether to go pick up my phone). Garmin's API/SDK looks pretty cool, though.
I looked at some devices a week or two when purchasing a new FitBit for the girlfriend but, like I said, they had loads of features I didn't want. I'm tempted to just continue to wait on purchasing a "smart watch" and get a new wristwatch instead. I'm not really thrilled at the idea of having to charge my watch all the time anyways.