I owned a couple, with peak Pebble being the Pebble Round. It was the only smartwatch I owned that understood it was a watch meant to be worn on the wrist, and not a portal into a series of invasive micro notifications. It was helpful, without being obnoxious. It was slim and stylish, the kind of thing you'd wear even without the built in intelligence. It was there, but only when you wanted it.
Also the battery life was measured in days, when Apple couldn't go more than 12 hours.
I had a Pebble Time Round, too, and it was great. For me the Pebble hit a sweet spot for a smart-enough watch that allowed me to keep my phone in my pocket. I could get notifications and reply to them right from the watch. I have a Withings SteelHR now, and it's great as a watch + activity tracker, but not being able to dictate replies to the notifications I get means I reach for my phone more now than I did with my Pebble
Also the battery life was measured in days, when Apple couldn't go more than 12 hours.