My 2 cents on this feature's "quality", reposted from:

https://support.zendesk.com/hc/en-us/articles/4591924111770-Using-lookup-relationship-fields

THIS:

"Unfortunately, I have to agree that this looks like a half-baked feature. Lots of potential but then (according to Zendesk support on using user lookup fields in views):

(...) you are not able to use end-user identities as a condition. Even if this is within a lookup relationship field with no filters on it. You are able to add a custom lookup relationship field to the columns for a custom view, but you cannot use that field to group or order the view (...)

So forget about having views that show tickets where in the lookup field you have end-user X. And forget about grouping or sorting views using those fields..."

AND ALSO:

"We have tons of situations where the ticket is pending on specific users of our Zendesk (approval, participation, clarification, execution, etc). And they may be agents or end-users. So I'm using a lookup field to allow the agents to associate each ticket with the user the topic is pending on and I also record the date. And that part works fine.

However, I need my agents to be able to create views where they have the tickets pending on specific users (one or more) and at this point we can use lookup fields as criteria but we can only search for agents. No good.

Additionally, even if we could specify one or more end-users as criteria, we can't use the lookup field to group or sort the view. If we have a view that lists a lot of tickets pending on a lot of different users, that would come handy. But we can't. Also no good.

On top of that, maybe we could use Explore to generate dashboards to take the place of the views we can't create. Big NO on that one as well.

My workaround: I have a parallel field (simple drop-down) where, using external API automations, I create an option for the user the ticket is pending on. When I change the lookup field, the automation updates the options in the parallel drop-down and updates the ticket with the correct option. Given that the simple drop-down does, in terms of views and Explore, everything the lookup field can't, the issue is resolved.

AND MORE RECENTLY:

If a lookup field has had a value in the past, that was cleared at some point, if it is being used as criteria in a view, it will always return TRUE if the filter is "fieldX is present", even if there's no value there...

C'mon Zendesk...