I've got a 17" model. It is plastic, but I have no complaints about the overall build. The keyboard has pretty nice long-travel keys. The trackpad is horrible, the cursor tends to jump around a lot when you try to click. This is my first linux laptop so I don't know whether that's a common driver issue on linux. If not for that I would be 100% happy.
Another model got a lot of complaints about the keyboard a year or two ago, and System76 responded by sending everyone a better one for free.
I've read (almost exclusively) bad things about the keyboard and overall plastic-ity of them.