The frozen ones barely taste like the ones you get at the restaurant.
They're different than most because they're steamed, which is kinda odd for a hamburger.
If you live in the south, Krystal's burgers are probably the closest thing to it you can find:
http://www.krystal.com/Also square, also steamed. They look and taste very similar. Both have been around forever... I think white castle is older though.
In-N-Out is better, so is Whataburger if you want an actual hamburger.