I really like painting with the line tool. I set it at a low opacity and a size that is bigger than I am comfortable with...sometimes a lot bigger. At first it feels clunky and unruly, but after a while it forces me to think of masses over lines, and to see values in a more orchestrated way. Its very fast to get an idea down this way, and it tends to leave behind a lot more unpredictable happy accidents of overlapping tones that can really texture a piece of art. I just paint darks and lights until the basic form feels sculpted. I highly recommend trying it.