Burger 21 is the real deal. I had the Tex-Mex Haystack Burger Medium rare (cooked perfectly) with fries ($7.99). It was awesome. My son had a Kids Cheesy Burger with fries and a chocolate milk ($5.49). I was impressed that the chocolate milk was made to order. It wasn't the stuff in the little milk cartons. Nice job. The restaurant was clean and the staff was very friendly. I know most everyone will compare it with Smashburger, Five Guys and Habit Burger. But here's some differences. First looking at Kids meals. My sons Cheeseburger, fries and made to order chocolate milk was $5.49. Smashburger it's $5.78 and that's with Horizon carton choc milk. Five Guys doesn't have a kid's burger so he gets the Little Cheeseburger ($5.09) which doesn't come with fries or chocolate milk. In fact, they don't have chocolate milk or really any drinks for kids other than the soda machine and tea. Regardless, adding $3.79 for fries (which of course is enough to share) and $2.39 for a regular drink and Five Guys becomes VERY expensive to take my son for lunch or dinner. In fact, just about everywhere is cheaper for a kids meal. And Habit Burger is the least expensive @ $4.50 for a kids cheeseburger, fries and choc milk.
The burger I ordered, the Tex-Mex Haystack @ $7.99 would cost about $7.29 at Smashburger, $8.99 at Five Guys and about $8.35 at Habit although Habit includes fries and a drink. They each have their own positives but if you're near Burger 21 I would recommend stopping and checking them out.