Amen to that!
I've been using a quilt journal since 2004 (just a small spiral-bound notebook) and it's saved my sanity more than once. When I thought I'd lost a block in a carefully numbered set, it was the quilt journal that reminded me that the series ended at 14, not 15! (That was a terror -- I tore the sewing room apart before thinking to check the plan.) You only have to do the finicky math for how much background fabric is needed for a quilt once - jot it down and look it up when you go back to a project. And it's great for ideas that come to you in the middle of the night and wake you up; write 'em down and go back to sleep, they'll be there in the morning and you won't forget!