But keep in mind that Dell is releasing their drivers into upstream kernels, so if you are on 14.04, you should be using the LTS Enablement stack in order to avail yourself of the latest firmware with stability improvements (this is how I fixed the Broadcom Null pointer bug).
https://wiki.ubuntu.com/Kernel/LTSEnablementStack
You don't have to use the LTS Enablement Stack if you installed with newer service pack .isos.
The 15" also has Thunderbolt which is a must if you want to have a reasonable docking solution for a Linux laptop.
Which, I really can't tell if it'll work or not. Really need to plug a 4k monitor into it.
From what I found out (if I remember correctly), I have great confidence that Thunderbolt docks should work well with Linux kernel 3.19+. But take this with a grain of salt since I ended up with the 13 DE and therefore only have a mDP and couldn't try a Thunderbolt dock.
One gripe so far: Since I updated the BIOS, the entire thing performs like a Netbook-Hype-era Atom CPU when there's no AC power.