System76 just rebrands Clevo laptops and installs Ubuntu on them, and host their own PPA to deal with system-specific drivers. In my experience, the laptops aren't any more linux-friendly than your average laptop (certainly less than your average Dell, or older Lenovo), it's just that you have, in theory, somebody else coming up with the hacks to get certain functionality to work, and pushing it to you as an update. Which is certainly something that many people might find appealing, but not everyone (especially those that like installing their own system).

Personally, I would just buy an old Thinkpad (see [0] for a great guide that was posted here a while ago) from before the time Lenovo became evil. I have an x201 that I rebuilt from cannibalised parts for less than $100 total (not including the SSD). An i5 and 6GB of RAM, 3 USB 3.0 ports added via ExpressCard and a 9 cell battery, all for 1/6 the cost of System76's new 'Lemur'. I have never experienced such wonderful Linux support out of the box. Although, you don't get a warranty :P

[0] http://ktgee.net/post/49423737148/thinkpad-guide

Edit: spelling