In my testing here is what I did and my results.
Using this CAML as a default:
Camlex.Query().Where(x => x["Trainee"] == (DataTypes.User)domain\username).ToString();
In my SharePoint list, the column Trainee is configured as a single person/group type.
When I set the "Show field" as Name I see LastName, FirstName in the list display, but my Caml returns 0 results
When I set the "Show field" as Account I see domain\Username in the list display, and my Caml returns 1 results, but I don't have the LastName, FirstName display that is more user friendly.
When I set the "Show field" as Name (with presence) I see LastName, FirstName in the list display, but my Caml returns 0 results
I am sure this is probably not a problem with Camlex.Net, but I am just trying to figure out how to query with login name and display with the Last, First. I don't want to have to create a duplicate column, as that would be a hack solution.