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Pro 2 day battery life
Despite it's very thin body, it gets 2-2.5 days of actual use. If you usually have the brightness maxed out however, that could dip down to just a day and a half. Either way, it will last for at least a full day as well as a night (if you wear your watch to bed).
The watch also charges very quickly - 15 minutes will yield about 60% of a charge, and it will be fully charged in 30 minutes. In addition, SnartStraps are expected sometime in 2016. This is a larger strap that includes an extra battery, allowing your Pebble Time Round to last up to a full week between charges.
Pro Small and lightweight
While most smartwatches are very bulky and not super comfortable to wear, the Pebble Time Round is only 7.5mm thick and weighs 28g. Compare this with the 10.5mm and 50g Apple Watch or 9.7mm and 62g LG G Watch R, and you'll see that it's really thin and light for a smartwatch.
Pro Battery efficient display
To save battery, Pebble went with an e-paper display. This means that the screen will only draw power when its refreshing the image, saving battery life. The e-paper display is the main reason how Pebble could make the watch so thin and light.
Pro Simple to use
On one side is a power button, and on the other side is a select button surrounded by up and down buttons. The Timeline UI is simple - press up to get to previous notifications, and down to scroll through upcoming calendar events or future events.
All navigation is done through the buttons, as there is no touch screen.
Con e-paper display
The display may be energy efficient, but it is low resolution (180x180) and pretty small at just 1". It's also hard to read outdoors because of how dim it is, and colors aren't as punchy as on OLED or IPS displays.
Con Huge bezels
The bezels are very large, even for a smartwatch (especially when compared next to a Huawei Watch or Moto 360). Some versions have numbers printed on the bezel which at least makes use of it, but then that also limits the watch faces you can use.
Con Limited app compatibility
The Pebble Time Round isn't compatible with all of Pebble's older apps which were designed for square watch faces. Also, because it's a propriety OS, it won't see the same level of developer support as Android Wear or Apple Watch OS.