Recs.
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Specs
Pros
Pro Modular design
Project Ara introduces the concept of interchangeable hardware parts to a phone. This would allow you to choose parts of the phone based on your personal preference, task priorities and situational needs. For example you could choose a longer battery over a better camera, or add a physical qwerty keyboard, or a solar panel. You can even swap the screen for another.
Pro Opens the door for never before seen use-cases for a phone
Project Ara would allow for using a phone in ways that simply don't make sense currently, like turning it into a projector, or a Geiger counter, or a clamshell gaming device. There could also be scientific, medical and industry specific modules, and many, many, more ways of using this device that are not possible currently.
Pro Print your module cases
3D Systems make 3D-printing equipment and have partnered with Project Ara to create a continuous 3D printer that can quickly print large amounts of Ara module enclosures. Going forward, it's possible that multiple part of a typical manufacturing process could be delegated to 3D printers.