I'd say the Challenge, the atmosphere/service and the shakes are what really sell this place. The wall is lined with pictures of people next to the cute (and dorky) stuffed burger prop indicating their status as people who survived the burger challenge. While I was there, a kid proudly posed for a pic after completing the children 1/2 lb. burger challenge.
I thought the burgers were pretty good. Not stupendous, but above "average" which is why this place gets 4 stars. Plus, you have so many options to mix and match burger toppings. I added sun dried tomatoes to mine for no additional cost, though it was 99 cents to have my onions sauteed. Hmm...
The shake was the highlight of this visit for me. I had strawberry cheesecake flavor. Ohhhh it was soo good. I'd come back just for that. Service was really friendly too, and I liked the cute 50s atmosphere. I'd say this is a great place for lunch and good if you have kids.