They opened one in Pasadena and I went there today. The burger is 50% ground beef and 50% bacon. NO JOKE.
Slater's 50/50 Food Menu - Home of the Famous 50/50 Burger
You can also create your own burger like at The Counter. There are some interesting options available. The fries come with bacon infused ketchup and there is bacon salt.
Next time I go back I'm trying the Guinness mint chocolate milkshake.