Have been using the 2nd gen Dell XPS 13 developer edition that comes pre-installed with Ubuntu and I'm loving it. Wasn't all smooth sailing though: http://www.sandeep.io/157
The latest edition is even better.
At the time I also looked at the ultrabook from System 76 but decided to skip it because of keyboard issue reports and lack of support in my country.
Edit 1: The keyboard on the Dell XPS is nowhere as good as a Thinkpad, but it beats them on form factor, price and support (at least in my country).
Edit 2: The X1 carbon has a slightly flexible carbon-fiber chassis which I didn't like but it's a great option if you want a bigger display than the XPS 13 and don't mind the price.
The latest edition is even better.
At the time I also looked at the ultrabook from System 76 but decided to skip it because of keyboard issue reports and lack of support in my country.
Edit 1: The keyboard on the Dell XPS is nowhere as good as a Thinkpad, but it beats them on form factor, price and support (at least in my country).
Edit 2: The X1 carbon has a slightly flexible carbon-fiber chassis which I didn't like but it's a great option if you want a bigger display than the XPS 13 and don't mind the price.