The prophet is reported to have said:
visits an 'Arraf (عَرّاف) and asks him
about anything, his prayers extending
to forty nights will not be accepted. (Sahih Muslim: 5540)
Going to an 'arraf (e.g., a diviner, seer or fortune-teller) is a clear violation of the Prophet (S)'s order, hence not allowed in Islamic Shari'ah. Since God is `Alimu-lghaib (Knower of the Unseen), visiting any 'arraf to gain knowledge of the unseen can also be considered a form of shirk. As God says in the Qur'an,
72:26: [He is] Knower of the unseen, and He does not disclose His [knowledge of the] unseen to anyone
72:27: Except whom He has approved of messengers, and indeed, He sends before each messenger and behind him observers
And as for anyone who claims to have knowledge of the future, God said:
31:34: Indeed, Allah [alone] has knowledge of the Hour and sends down the rain and knows what is in the wombs. And no soul perceives what it will earn tomorrow, and no soul perceives in what land it will die. Indeed, Allah is Knowing and Acquainted.
So it seems clear that anyone who goes to an astrologer for the purposes of knowing and predicting the future has strayed from the straight path.
That said, I know of no strong evidences against using astrology for purposes other than discerning the unseen.
For example, there are many astrologers who claim, through (pseudo-)scientific observations made over generations, that the positions of the stars may influence your personality or behaviour; rather than claiming to know the secrets of the unseen, which are God's alone to disclose, they are trying to explain observable phenomena using other observable phenomena.
Even if there's nothing to forbid such practices, or following those who espouse them, refraining from such is still probably safer since astrology (in general) is so strongly correllated to fortune-telling.