The information provided here is based on many months of practical experience using Stevia as a sugar substitute.
The only way I buy Stevia is as the pure white powdered extract. You can also buy a green extract (which is simply the ground Stevia leaves) and liquid extracts. My experience is that you get the best sweetening power with the least aftertaste using the pure white powdered extract. The brand I use is called "Now" which is available in my local health food store. It costs me $9.49 for a one ounce bottle (I'm sure it can be had cheaper elsewhere) but that bottle lasts a LONG time. My guess is that even at that price it is competitive with other artificial sweeteners.
I normally use Stevia in conjunction with other artificial sweeteners - I keep a box of "Sweet One" around because I want a sweetener that will take heat and not break down. Stevia itself is known to be stable up to about 400F. The reason I do this is because combining sweeteners seems to give better results. Thanks to Dr. Atkins for clueing me into this tidbit.
There are a several practical problems with using Stevia as a sweetener. First, since the FDA refuses to approve its use as a sweetener (despite it being safely and successfully used in Japan for that purpose for 25 years) and only as a "food additive" the manufacturers cannot legally put a sweetening equivalent on the label. Second, the sweetening power of the pure white extract is 200 to 300 times that of sugar. Inconsistency of that magnitude makes it important that once you find a manufacturer you like, you stick with them. Third, because it is SO sweet, using the pure powder alone is difficult to impossible.
These problems can be overcome to a great degree by making a liquid form of Stevia from the pure powdered extract. I got the formula for this from the "Paul on Sweeteners" web page. To do this, slightly warm 2 tablespoons of water (filtered or distilled is best) in the microwave. Add 2 teaspoons of white Stevia powder and stir until dissolved. It will take a minute or two until it completely dissolves. Store in a tightly closed container in the refrigerator - I use an old spice jar that has been thoroughly cleaned. This keeps in my refregerator for weeks. (By the way, this process can get a little messy. Static electricity sometimes makes the powder go places you don't want it to go. Expect a little waste).
One teaspoon of this liquid is the equivalent of approximately 3/4 cup of sugar or about 9 packets of artificial sweetener. As a rule of thumb, however, I substitute 1/8 tsp of homebrewed Stevia liquid for each packet of artificial sweetener. So, for the Atkins cheesecake, which calls for 12 packets of artificial sweetener, I normally use 1 tsp of Stevia liquid and 4 packets of Sweet One. You could simply use 1 1/2 tsp of the liquid, but I find that the mixture tastes better. For the Atkins fudge, which calls for 4 packets of artificial sweetener in addition to the aspartame in the pudding mix, I substitute 1/2 tsp of Stevia liquid. This works quite well. For a glass of iced tea or a cup of coffee, 1/8 tsp is fine (your tastes may vary).
There are various web pages that have more information on Stevia itself. If you are interested, go to the "Paul on Sweeteners" page - the links of interest are at the bottom of the page.