I started using one last year as part of my New Year's quilting goals and I have to say, there's a sense of relief and accomplishment that followed.
I write every detail I can down, how much yardage, who the manufacturer of the fabric is, is it part of a collection, how much I paid for it. Thread is done the same way, if I use acrylic templates, I write that down, too. The kind of batting I'm going to use, what size backing I want to use, what size the quilt should be when I'm done, what size the block should be when I stitch it together. If it's a pattern I'm following and I make adjustments, they get written down.
Since I buy everything online, every receipt is printed off and put into the journal, too.
While I'm making the quilt, I post pictures on the person's Facebook page so they can see it as I make it. Once the quilt is done, I'll print those pictures off and then add it to the journal and close out that section.
Since I'm going to start free motion quilting my own projects, the patterns I use will be documented, whether or not its a stencil or a ruler and weight of the thread. I figure the more I write down, when I'm no longer on this Earth and my family takes possession of it, they can see what I've done and if need be, track down everything in the event something happens to one of their quilts and they can either restore it or have it re-made.