I used to work in the area so I can only speak for lunch time meals but the Frontier Restaurant/Diner is a decent l'il diner in the Midtown East area.
A diner is a diner is a diner. The Frontier has a whole cowboy-theme it seems to have started with when it first opened, but it is really just your typical NYC Greek diner with decent food, big portions and good prices. There might be some lassos or boots drawn on the menu or on the wall, but I can't really remember.
It's never empty nor is it too crowded. It's not especially dirty nor is it exceptionally clean. It's just there and makes for a dependable place to get your typical diner fare. I think the only reason I remember is because it has the whole cowboy/wrangler theme going on.