I have used spells that others have written, but I've also used my own spells. I'm pretty good at it, and the words just seem to flow out.
For instance, I just wrote a spell for my legal case, and I think it's really good. I am anxious to try it out, and it will be going into my Book of Shadows, final copy.
Does spell writing come easy to anyone else, or do most people use pre-written spells?