If you're happy with your mattress, you probably aren't reading this, so I'll assume your mattress is either causing you some back pain or you're feeling the pressure points that happen when your body isn't quite suited to what you're sleeping on. In those situations, provided your mattress isn't actually sagging or needing to be replaced, a high-quality mattress topper suited to your body and manner of sleeping can indeed help.
Keep in mind that there really is no best mattress topper; there's the best mattress topper for you. A topper that's great for back sleepers might be too firm for side sleepers, and stomach sleepers might need something different. For those who sleep every which way, like me, there is both more leeway and less, since a topper needs to be comfortable in every position. The best way to find the perfect topper for you is to match the sleep surface to your most used sleeping position. When those two things align, you'll find pressure relief and start sleeping better and waking up more refreshed.
Mattress toppers are generally best used on mattresses that don't have any kind of topper. In other words, you're adding the pillow top. If you already have a pillow top as part of your mattress, adding a topper will probably end up being little too much.