Stevia has a mild after taste of its own when compared to Sugar. It is true for almost all the sweetners that they have their own taste which is different than Sugar. Many stevia based seewteners also have after taste but there are many stevia extract are available in market which does not have any after taste, though they are a bit costlier, and they are as safe as stevia or stevia extract with after taste.

Even the after taste or the bitter taste of stevia or stevia extract is different than the taste of suger but it in itself is nota bad taste and you will like it once you start using it regularly.