Stevioside (the sweet chemical in stevia) is 300 times sweeter than sugar.
In Europe it is forbidden because of pressure from Nutrasweet and other untested artificial sweetener producers.
It's very sweet, all-natural and a lot better (health-wise) than sugar or aspartame, in my opinion. But depending on the brand you get, it can have a bitter aftertaste (don't buy the "Now" brand). It's a really fine powder, way finer than sugar so sometimes it's a bit of a pain if you're trying to mix it, it gets all in the air and you end up inhaling it.
The best tasting brand I found was SweetLeaf Stevia with FOS.