Yesterday, I've upgraded my Nexus 5 from SFOS 3.3 to 3.4. (Thanks for @mautz for creating a repo for this.) As the upgrade was a bit unusual, I thought to share my experience.

I had to repeat the upgrade about 5 times because every time I rebooted into the new system and reached the lock screen, a few seconds later the screen went black. The phone was accessible via ssh, just the screen remained black.
When that happened I deleted that from, copied the other working one and tried to upgrade again.
Finally, I identified that if I didn't let the android support to start at boot, that solved this issue.
As you may have other problems after the upgrade, I highly recommend doing a backup before you start.

The upgrade steps are defined in this post. I followed that with the following little tweaks:

• DO A BACKUP!!! In TWRP or if you use multirom, create a copy of your system and try upgrading that one!
• uninstall all patches and patchmanager
• uninstall aliendalvik-control (Actually, somehow I had two versions installed. The first one was removed fine with pkcon, but the second one has dropped an error. Finally, I removed that with this command: 'rpm -e --noscripts aliendalvik-control')
• disable to start android support at the boot!
• disable all openrepos
• disable all adaptation-community-<something> repos
• add/enable the following 2 repos:
  
  Code:

  ssu ar adaptation-community-common-hammerhead http://repo.merproject.org/obs/nemo:/testing:/hw:/common/sailfishos_3.4.0.24/
  
  ssu ar adaptation-community-hammerhead http://repo.merproject.org/obs/home:/mautz:/cm12.1/sailfishos_3.4.0.24_latest_armv7hl/

Some benefits of upgrading to 3.4:

• newer and much better browser engine
• camera viewer works again
• user 'nemo' will remain, so AD works without any further tweaking