I used to work at Culvers in high school...the biggest selling point is the huge menu and the ability to customize. All of the employees and many regular customers had their own custom menu items. You can add anything to a grilled cheese, for instance, and they have a number of types of bread and cheese to choose from and a good selection of high quality sauces. I don't know if it's still the case, but we could do most of the 'limited time' stuff on request even if it wasn't on the menu at the time. I usually ate various forms of a turkey melt, I can't even remember what the deli turkey was actually for haha.
The cheese curds were good 20 years ago (has it really been 20 years since I worked there...ugggh), but are pretty mediocre by today's standards. Maybe it was just my pickiness but I also thought there were not many procedures in place to ensure consistency, so it wasn't uncommon for us to serve overcooked burgers.
These days when I go back to Milwaukee I go straight from the airport to Kopps on 76th and Layton.