I was always taught, the best learning is achieved at about 70% success rate.
Too high success rate indicates one is not facing any difficulty, and is not learning to improve beyond his current ability.
Too low success rate does not leave sufficient good examples to follow.
(I did not vote because such an option is missing in the poll)
Yes, I agree. Sucess and failure are equally important. One without the other makes it impossible to progress, which is why I didn't vote, either.
Interesting question, tho. And, I like Mark Uttech's response!