I think it depends a lot on how good you are at the subject beforehand, but yes they can be very, very important, especially regarding your enjoyment of the subject. For Biology, I had one amazing teacher and one "eh" teacher. As a result, I came to hate the parts of the course that the "eh" teacher taught, and found one of the two topics they taught me the hardest.
On the other hand, I had an amazing teacher for Physics. Although I was awful at it, I chose it mostly because she was so incredible, and I loved my year in the subject, and I'm reasonably sure I would not have been able to even consider getting above a U without her.