I absolutely do find this to be unethical, and I consider it cheating personally. Many professors reuse exam questions. I had one professor in college who was like this. By looking at old exams, you are obtaining an unfair advantage over your classmates, and if there is a curve, you are skewing it against other classmates who obtained their grades without an unfair advantage. At my school at least, a professor could easily report you and you would be expelled for this. The only time I think it would be fair game is if you asked the professor ahead of time, and even then I would personally feel badly about the advantage unless the materials were in the public domain.