Usually in textbooks there are some summaries. I usually just photocopy these (especially the ones to do with graphs) and add a few examples here and there about things i'm not too sure on. Ta da! There's your notebook (with some prac exams at the back).
I like to make a notebook up as a security blanket, as someone said earlier. I remember in Year 10, i spent ages at the end of the year making up a notes book from scratch. The good thing that came out of it was that while you are making it, you realise whether you know the course or not: if you wave it off and don't put it in, it means you're confident on that topic; if you hesitate and put it in your notes as a back-up plan, then you know that you don't know that topic as best as you possibly can. So, making notes was a different form of revision: revising knowledge that may have been hidden in the back of your mind.
The obvious downfall to making notes (at the end of the year, from SCRATCH) is that you waste valuable, valuable time. It may all seem like a waste at the end of the exam, when you realise you didn't even open it, but the value in that is that YOU KNOW YOUR WORK. So, some see it as an investment; others, a waste of time.