The fresh and dried leaves are about 15 times sweeter than sugar, while extracts made from stevia are roughly 300 times as sweet as table sugar. Some people say it has a licorice-like or woody aftertaste, but otherwise it just tastes sweet. Healthwise, raw stevia leaves contain just a few calories and the refined commercial products are virtually calorie free. It also seems that stevia is absorbed by the body more slowly, so it has less of an affect on blood glucose levels than sugar.
Some health concerns have been raised over the years. Studies were conducted in the 1980's that suggested that stevia might be carcinogenic and cause fertility problems, but the evidence has remained inconclusive. In 2008, the FDA cleared purified stevia extracts (specifically rebaudioside A) as being "generally regarded as safe." The leaf can still only be sold as a dietary supplement.
Stevia is most commonly used to add a little sweetness to things like a mug of coffee, salad dressings, or a bowl of yogurt. We haven't used it much in baking so far and we know the conversion from sugar to stevia can be tricky, but it's certainly possible.
For further information, take a look at these links:
What are your thoughts on this sugar alternative?
Related: Agave Nectar: Healthful or Harmful?