I got an ASUS U31JG (13" non-ultrabook) about 5 years ago and added RAM and an SSD and it is still going strong (and nice to have HDMI and VGA). I think the Zenbooks evolved out of that line so an ASUS Zenbook (UX305) will be my likely replacement. Although I have also been eyeing the XPS13. It will likely be between XPS13 and UX305(FA or CA). But like I said, this 5 year old ASUS (with a few upgrades) is still chugging along.

Same here, great experience with Linux on UX305FA. I switched to multi-boot setup, shrinking Windows partition and giving Ubuntu 14.04 half of the 256GB SSD. Switched window manager to i3, which is excellent, can't imagine going back to floating windows after i3. UX305FA is supposed to be a relatively slow processor, but most everything seems snappy to me, and power handling is excellent (with TLP package).

Also, UX305FA is fanless. I love having no fan, hope to never again have a laptop with a fan.

If you're not trying to do anything too taxing (not planning on playing games):

ASUS Zenbook UX305FA-ASM1 model M (the fanless processor) gives me great battery life, and works nicely with Arch.

Thought I'd add, I love my UX51VZ three years later.

And updated UX305CA works great with a 4.5 kernel.