The are a number of narrations indicating that the Imams (A) had comprehensive knowledge of astrology, and you can find them in books such as volume 55 of Bihar al-Anwar. (Whether one accepts them is a personal decision, but there are no narrations to the contrary.) One narration (the "Myrobalan Fruit") implies that astrology exists as a way to prove the existence of God and the prophets' knowledge from God. 

Whether or not astrology is considered shirk has tended to depend on worldview. If someone is an atheist and believes that there is no God but that the stars and planets control their destiny, then this is shirk. If someone believes that their fate is wholly fixed by astrology, and that God cannot change it, this is also theologically inappropriate.

If someone believes that God is in command of the entire universe, and astrology is part of the divine plan, and that our ultimate fate is in the hands of God, this is not shirk.

Some Muslims today are too eager to throw around the word "shirk" without thinking about what they are actually saying.