According to the label on a miswak I bought:
Before the advent of plastic, synthetic toothbrushes, people in many parts of the world cleaned their teeth with wood from the all natural Toothbrush tree. Today, many still use Miswak as their primary preventative measure in oral hygeine.

In Islamic tradition, the Prophet Muhammad (pbuh) said: "Make a regular practice of Miswak, for verily it is the purification for the mouth and a means of pleasure of the Lord" [Al-Bukhari]

Potential benefits with Regular use: Research shows that the bark of the "Toothbrush tree" contains an antibiotic which suppresses the growth of bacteria and the formation of plaque in the mouth*. Research also suggests that the regular use of the miswak signifigantly reduces plaque, gingivitis, and the growth of carcinogenic bacteria* No toothpaste required! Miswak naturally contains many components such as fluoride, astringents, detergents, resins (a possible enamel protectant) and abrasives.*

Easy to Use: Simply scrape off bark from the tip (1/2"), then chew the tip gently until brush-like and the fibers become soft. Brush teeth horizontally and frequently. When the bristles are worn and the flavor has subsided, cut them off & repeat instructions.

* These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any disease.

mr100percent's experience: It's not bad. You have to soak the tip in water for a few hours for it to become soft and toothbrush-y (something the directions didn't mention, but everyone told me to do). It's got a wooden but natural flavor, and does a pretty good job at cleaning. It's a little bit coarser than a regular plastic toothbrush, but after getting used to it, I use it even more frequently than a regular toothbrush. Most people carry it in their pocket, it doesn't need water or spitting, so you can brush anywhere really. It's quite common in Africa, South Asia, and the Middle East. Since it grows on trees (though the shopkeeper once told me that I got a root instead of a branch), it's very cheap, expendable, and thus the perfect toothbrush for much of the world.

Links (to buy and find more info):