I use it when I get stuck, or or to learn how to do problems that the professor is doing a horrible job of teaching. I would never use it to copy entire solutions down and pass the work off as my own. That's just shooting yourself in the foot. If you're using it like a tutor - to help you learn the material - it's not cheating at all. Besides, if you "cheat" on every homework assignment and never learn the material, you'd end up failing the class.

I once had an instructor call me out because he recognized the form of one of my answers as being from a "solutions manual". It was a problem I was having a very hard time with, and spent many hours on. By the time I was done, I understood the problem and how to solve other problems like it. He called me out in front of the class, and basically threatened to report me for academic dishonesty. I found it a little bit odd because homework consists of maybe 15% of our grade, and is essentially there for us to learn the material, not to test us on it.