Well, I would argue that EVERY settings adjustment in Camera Raw/Lightroom can be overdone...but that's not a reason to not use them.
Most images will show an improvement when applying between 10-30 as Mike says...a lot of it is dicatated by the image. If you have a lot of medium frequency detail in the midtones, Clarity will have a strong impact even at lower amounts. There are some images where even at 100 you hardly see an effect. The more midtones your image has, the more impact the Clarity adjustment will have. But, as with ALL CR/LR adjustments, use them wisely...less is often more.