I do like sketching in biro, you can get a good strong line, but by varying the pressure on the pen its good for shading. I have also found that by spraying the biro with hairspray you can sometimes get the ink to blur and bleed. If you are using black or blue biro you get interesting colours flowing from the lines. I guess it depends on the chemicals in both the pen and the hairspray. It might not always work. I have also used hairspray as a fixative for astel and charcoal drawings.
Did you know you can use milk sprayed onto charcoal or pastel as a fixative too? I found an old perfume bottle with an atomiser and used that to spray drawings. They might end up smelling a bit cheesy but it works. You just have to remember to wash out the bottle and spray after use.