I use it all the time. It is great! Right now I have a queen size quilt on my frame. I am doing a small design in each of the big squares and then stitching in the ditch on the rest. I mark the design with the chalk as I go along, one at a time.
The chalk will break fairly easily if you press too hard. However, refills are available at LQS. They are in mine at least. The name of this product escapes me at the moment.
Don't be afraid to use it. As I stitched it just about disappeared on its own and I brushed it out with my hand the rest of the way when I was done. I am working on 100% cotton. Test a small corner somewhere to be sure for your project though.