First off, diners are good places to eat. The food might not be cordon bleu, but it’s always solid and you’re not paying a premium for presentation and arrangement. Wherever I travel in the US, I always check for diners first.
So, how does Smyrna Diner stand up? Just fine. I’m comfortable eating there, they make me feel welcome, they have plenty of food for restricted diets (gluten free buns for hamburgers! Wonderful!), and there’s lots of space to fit visitors. I would happily eat here for every meal.