There are many problems with Optimus, but currently two main problems prevent me from logging in.
First; installing nvidia drivers via official repositories results in a blank screen during login. Purging nvidia seems to solve this.
Second; without nvidia drivers my nvidia VGA appears as a "secondary display". Its resolution is much smaller than it should be. I can't find configuration files to modify those values.
I need to find a way to use nvidia drivers and resize that "secondary display". I'm currently installing Bumblebee to see if it still works.
EDIT: using bumblebee gives following error "Cannot access secondary GPU, secondary X is not active."
EDIT2: following this guide (with some changes) http://tuxilero.blogspot.cz/2013/09/how-to-official-nvidia-drivers.html I was able to enable nvidia and now the only (minor) problem is that some features like hotkeys and themes aren't working.
BTW: I'm using Gnome 3 and GDM. Unity and LightDM just show the error message asking for further actions (such as run in low-graph environment or restore default settings).