It really depends on the subject in my opinion, through writing notes you are laying out previously studied information when you understand it in a way which you can more easily understand it (also known as "writing in your own words") for later reference. I know a lot of people that just write notes straight from the textbook without gaining an understanding of the concepts behind the information, and subsequently notes don't do them any good. However, having the information laid out in your own words once you understand the concepts makes studying much easier
Notes are not for everyone though, The dux of my school (98) last year didn't make notes for anything.
I myself see notes for subjects such as Business Studies as absolutely essential due to the huge amount of content you need to get through, however with subjects like IPT i prefer to do regular revision of class made notes and papers over traditional syllabus notes, however syllabus notes do help
If you want to use others notes, go through the textbook and understand the concepts first, followed by using their notes as a study aid is my suggestion