Pure stevia should have no carbs in it, nor should it have calories. Besides, sugar has lots of additives and processing that are bad for one's health beyond the mere calories and carbs. So it is a much better alternative to sugar.
Stevia is your best choice for sweetening products.
Stevia is the only calorie free sweetener that is naturally derived. The only thing that sucks about stevia is that it's way more expensive then artificial sweeteners like splenda, aspartame etc.
As with everything, moderation is key even with good things. So, even though there are no specific side effects with stevia, I don't think it is good to consume it in excess.