It is very interesting to see how few people regards Schopenhauer as a great thinker.
Obviously he is generally undervalued in society and especially among university philosophers. The reason for this in my opinion is that he crushed so elegantly every charlatan in philosophy that a lot of them are and were scared because reminded of their own worthlessness comparing to his great achievements in the field of metaphysics they took refuge in ignoring simple and elegant truths he produced.
He was a true genius. It is important to remind that to enable him to become one of the greatest a certain dispositions were required.
First of all after his father's death he inherited a large amount of money which enabled him to gain financial independence. So he was not obliged to earn money for his living and could devote all his time to study.
Secondly, in his teachings his standard was to speak and follow only and nothing but the truth despite the consequences which a way like that could bring.
Thirdly, possesing unparalleled beautiful and elegant style of his prose he was able to present his philosophy in clear and concise manner which is univerally recognized even from those who do not agree with his philosophy.