"The Granddaddy Of All Root Beers", is what Sioux City Sarsaparilla claims to be. I picked up a 4 pair of this soda pop at Horrock's in Lansing, MI.
So what exactly is sarsaparilla? It is a vine that is native to Central America. It was popular in the old west to flavor carbonated water. There is a cowboy entering a saloon on the bottle, which is an old to sarsaparilla's history. It's hard to find nowadays. Sioux City is the most popular brand. The easiest place to find it is at tourist stops, especially those related to the Old West.
Sarsparilla tastes a lot like root beer, and so does the Sioux City variety. It almost has a sweeter flavor to it. It's different enough from root beer to have it's own name, but I think anyone that likes root beer would love this. If you can get your hands on a bottle or two, I recommend it. It's good!
Check out this review of Sioux City Berry Berry.