They have arrived with winter20. And they are tricky! Logging my initial thoughts and experiences to something I’ve been really wanting for years.
You can’t directly reference an object, like i want to do a lookup to campaign.
You have to reference an existing lookup field on a child object, like I want to lookup Campaign from Campaign Member, or I want to lookup Accounts on Opportunity.
As far as i can tell the end result is the same - you get the record ID, and you get the record name.
It looks like getting any other fields requires you to do a separate get records step, but thats easy enough.
See the image from the release notes - its looking up account from opportunity.

Here is a screenshot of my lookup to campaign

RecordId allows you to pass in a default value.
For more details, which Brian Kwong’s video here
My use cases
– i use a flow to create contacts, will be...