I've just installed only Ubuntu 14.04 on my C720 after previously having both chromeos and Chrubuntu installed.  It was reasoably painless, although the process required several steps contained in several posts and on various web sites.  I've collected the process I used into a step by step process (in case I need to use it again) so I thought I'd post it here.  Note, I don't take any credit for the actual steps involved, all I have done is collect them together.  Anyway, here it is:

Download 64 bit version of Ubuntu 14.04
Create bootable USB flash drive containing Ubuntu, e.g. on a Windows PC use Unvversal USB Installer

Remove the write lock screw:
http://www.chromium.org/chromium-os/developer-information-for-chrome-os-devices/acer-c720-chromebook

Close the back cover with only the battery lock screw.

On boot it will complain that chromeos is broken (unless your C720 already contains Chrubuntu, then it should just display the usual developer screen), so iIf

necessary, reinstall chrome os from factory with an usb stick:
https://support.google.com/chromebook/answer/1080595

Boot into chromeos, go to a terminal (ctrl+alt+ ->) and login as chronos.
Then type:
    sudo su

Set friendly bios flags by typing:
    /usr/bin/set_gbb_flags.sh 0x489
    crossystem dev_boot_usb=1 dev_boot_legacy=1

Power off the chromebook, open the back cover, and re-install the lock screw, then close up the case.

Insert the Ubuntu USB drive, connect a USB mouse (since the touchpad won't work at this stage), and power on. Note that it will take a while to boot (slow boot

issue which we will fix below)

Choose to install Ubuntu and have it wipe the disk as part of the installation.

Once it is installed, restart, remove the USB drive and it should boot Ubuntu. Again the boot will be slow.

Fix the slow boot issue by installing the 3.14.1 kernel, as follows.
Open a terminal (ctrl-alt-t) and type the following:
    wget -c kernel.ubuntu.com/~kernel-ppa/mainline/v3.14.1-trusty/linux-headers-3.14.1-031401_3.14.1-031401.201404141220_all.deb     wget -c kernel.ubuntu.com/~kernel-ppa/mainline/v3.14.1-trusty/linux-headers-3.14.1-031401-generic_3.14.1-031401.201404141220_amd64.deb     wget -c kernel.ubuntu.com/~kernel-ppa/mainline/v3.14.1-trusty/linux-image-3.14.1-031401-generic_3.14.1-031401.201404141220_amd64.deb     sudo dpkg -i linux-headers-3.14.1* linux-image-3.14.1* .deb

Before we restart, lets fix the suspend issue now.
First create the 05_Sound file under /etc/pm/sleep.d/ by typing the following:
    sudo gedit /etc/pm/sleep.d/05_sound
Into the gedit window type/paste the following lines:
    #!/bin/sh
    # File: "/etc/pm/sleep.d/05_sound".
    case "${1}" in
    hibernate|suspend)
    # Unbind ehci for preventing error
    echo -n "0000:00:1d.0" | tee /sys/bus/pci/drivers/ehci-pci/unbind
    # Unbind snd_hda_intel for sound
    echo -n "0000:00:1b.0" | tee /sys/bus/pci/drivers/snd_hda_intel/unbind
    echo -n "0000:00:03.0" | tee /sys/bus/pci/drivers/snd_hda_intel/unbind
    ;;
    resume|thaw)
    # Bind ehci for preventing error
    echo -n "0000:00:1d.0" | tee /sys/bus/pci/drivers/ehci-pci/bind
    # Bind snd_hda_intel for sound
    echo -n "0000:00:1b.0" | tee /sys/bus/pci/drivers/snd_hda_intel/bind
    echo -n "0000:00:03.0" | tee /sys/bus/pci/drivers/snd_hda_intel/bind
    ;;
    esac

Save the file, exit gedit, and make the file executable by typing:
    sudo chmod +x /etc/pm/sleep.d/05_sound

Now edit the rc.local file by typing:
    sudo gedit /etc/rc.local
Into the gedit window type/paste the following lines above the line that says "exit 0":
    echo EHCI > /proc/acpi/wakeup
    echo HDEF > /proc/acpi/wakeup
    echo XHCI > /proc/acpi/wakeup
    echo LID0 > /proc/acpi/wakeup
    echo TPAD > /proc/acpi/wakeup
    echo TSCR > /proc/acpi/wakeup
    echo 300 > /sys/class/backlight/intel_backlight/brightness
    rfkill block bluetooth
    /etc/init.d/bluetooth stop
Save the file and exit gedit.

Now edit the grub file by typing:
    sudo gedit /etc/default/grub
Edit the line that says:
    GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
So that it reads:
    GRUB_CMDLINE_LINUX_DEFAULT="quiet splash tpm_tis.force=1"
Save the file and exit gedit. Then update grub by typing the following two lines:
    sudo update-grub
    sudo update-grub2

Reboot now, by typing:
    sudo reboot

Finally we need to fix the touch pad issue.
Either download the following file, or just create a new file (open a terminal and type gedit install.sh) and paste the contents of the file into your file:
    https://raw.githubusercontent.com/raouldc/c720-touchpad-patches/master/install.sh
Edit the file and replace all of the instances of 3.13.4 with 3.14.1

Now run the file:
    sudo sh install.sh
There will be two patch conflicts, just ignore those.

When it has finished, reboot.

You should now have Ubuntu 14.04 with quick boot, working touchpad, and suspend.

Enjoy :-)
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