So it's finally here and I've been having a play. On long forms a user can now save their progress without the form closing and them having to re open it.
A couple of things to note. If it's a new item and you use save and continue, it creates it as normal and any required fields will still be required.
Workflows. If you have a workflow that updates the item on modify, you will get a save conflict warning. You can simply get around this by perhaps using conditional rules or have your workflow do nothing based on the input into a field.
In the below example the Save your progress button isn't connected to anything, while the Approve and Reject items input Process or ProcessReject into a workflow field. When the workflow runs on modify, it'll do nothing if the save your progress button is pressed.
So in a nutshell you may have to review your workflows before enabling this option.