Johnnie's Beef's motto is, "The Best Beef in the World," and I concur. I grew up not too far from Johnnie's and spent 36 years in the city. I've tried many Italian beef joints and many Italian beefs. Some are very good, but Johnnie's is a cut (or two) above all other beefs that I've had.
I've never had a piece of fat or gristle on their beef. The juice/gravy is outstanding! I personally prefer the combo: Italian beef with Italian sausage, which has been grilled to perfection over charcoal. I always order hot, sweet, and juicy (interpretation: giardiniera, sweet bell peppers that have been cooked/softened, and the sandwich has been dipped). I'm telling you, "heaven" comes to mind when you take a bite.
The restaurant is located in an old Italian neighborhood with a lot of history. The ambiance is very "Chicago." I don't know who is currently the cashier, but they've always have a "barely patient" person, who is quick, knows his stuff, but wants to keep the line moving (which is usually fairly long out the door). Personally, standing in the line smelling the charcoal-grilled sausage just builds one's anticipation for the food.
The Italian ice is exceptionally good! It provides a great refreshment during summer, and is very light, so it's good after you've stuffed yourself full of Johnnie's specialty: Italian beef.
In summary, from the line, to the smells, to the people working there, it all adds up to a good, authentic Chicago experience (Chicago and Elmwood Park are closely related). If I was a tour guide and was recommending restaurants and sights to see in Chicago, this would be one of them.