Pro Water proof
While it can't deal with deep water pressure, you have nothing to worry about when washing your hands or getting it wet in the rain thanks to the IP68 rating. You can also wash the band and unit off to remove any dust or debris that can accumulate on the underside of the central unit or in the cracks between the unit and the band without worrying at all about water damage.
Pro Transflective display
The display on the Smartwatch 3 doesn't require a backlight. Instead, it reflects ambient light. This makes it easily visible in ambient mode even when outside in direct sunlight (as a matter of fact, that's when it is easiest to see in ambient mode), and also saves a lot of power in ambient mode.
In a dark room, you won't be able to see the time...although this can also be counted as a pro because your wrist won't glow in dark rooms like OLED or IPS watches will.
Pro Running companion
The built-in GPS will drain the battery quickly, but it allows you to track activity more precisely. If you go on walks or runs, apps can track them without needing to bring your phone.
Android Wear lets you sync some music onto the watch, which can be paired to bluetooth earbuds/headphones. This fully lets you leave your phone at home, and use the watch for GPS and music at the same time.
Pro Uses a regular charger
You can charge the SmartWatch 3 using the same charger that you use to charge your phone - micro USB. This means that if you brake/lose your charger, it is easy and cheap to replace. It also means that if you go traveling, you can only bring one charger instead of a different one for your phone and watch.