You have to turn off Intel Smart Response (switch to AHCI), to even get it to boot from dvd on one of these machines.
You need to turn off "Rapid start" technology or suspend will resume automatically in Ubuntu, and use up the battery.
Installation is mostly like any other computer, it runs 13.10 just fine. You can manually make a system partition on the SSD, and a home directory on the hard drive during the setup.
See also: https://wiki.ubuntu.com/HardwareSupport/Machines/Laptops/Dell/XPS/L521X#preview
It looks like the only no-go is the Bluetooth, at least in Kernel 3.11 until the drivers improve.
Update - I have since updated to 14.04 and bluetooth and everything works.
A. Keeping all Ubuntu files in one partition is preferred; when you need to reinstall, you can back up, then verify, all the files under home before reinstallation.
B. I would keep the 128 GB SD Card separate from the root partition of Ubuntu, so you can easily access it from Windows, and remove it if needed. They have a significantly higher failure rate than SSDs or HDDs, and I would not trust a filesystem split between an SD card and another drive.
You can access files for read, write, create, and delete in the Windows NTFS partition if you wish to share files across both operating systems; however, Windows does not access files in Linux filesystems well. Every time I have tried to access a Linux ext4 partition from Windows using drivers, I have ended up with a corrupted filesystem.
C. RedHat says 4 GB or more for swap. Swap partitions are passé in Ubuntu since before 18.04 LTS; the default now is a swap file equal in size to your RAM, so you do not need a separate swap partition, and since Ubuntu is much more efficient in the use of swap. dphys-swapfile allows autogeneration of the swapfile as well as turning it off when no longer needed. Here's a good primer on swap and zswap, an alternative which uses CPU and memory instead of disk space if you've got a fair amount of RAM and processor power available. zram is also worth considering.
D. When you do the dual boot 'alongside' install, the Ubuntu installation app, Ubiquity, will ask you how much space you will take on the SSD for Ubuntu. If it's occupied be a Windows-style NTFS partition, Ubiquity will resize that NTFS partition and make space, whatever you specify, in the SSD for Ubuntu. You can also, later on, reallocate space for either partition, if you boot from the Ubuntu LiveUSB and run gparted.
Best Answer
Dual booting with Windows is a bit tricky, but it can be done. Lost a day to do it :)
Here are several resources that helped me:
I resized Window's partition using Window's disk utility.
Check that you have an up-to-date BIOS. I have A05. Version A06 is imminent and will be a very welcome upgrade (solves a keyboard problem which triggers repeaded keys).
I decided to encrypt the ubuntu installation, so I created one partition for boot and a physical space for encryption with a '/' partition over it. No swap: I added it later as a swapfile, instead of a dedicated partition.
I recommend Ubuntu 15.04 instead of 14.10, because the kernel is newer and wireless works out of the box (it will send your kernel to panic from time to time though... the dreaded caps lock led flash! UPDATE: fix available here https://launchpad.net/~inaddy/+archive/ubuntu/lp1415880).
I found the kubuntu installer would not handle full disk encryption correctly, so I installed with Ubuntu installer, and added the kubuntu-desktop package later. I had serious problems with LightDM (it would appear and immediately disappear after boot, also it would not allow me to switch VTs), so I switched to KDE's sddm and its login manager (hint: sudo dpkg-reconfigure lightdm).
Sound may not work work: you need to cold-boot TWICE (turn off and turn on again, reboot will not work!) every time you return from Windows. Same thing goes the other way (when returning from Linux to Windows). I believe this will be fixed in a future kernel upgrade.
Good luck with tiny fonts. But it can be fixed.
I am writing this from Kubuntu on XPS 13 which I use 12 hours a day for work. After a few days of panicking about supposedly broken things, I managed to have everything working and I am very happy with the purchase :)