I also picked up some brown ink on the weekend. http://www.sennelier.fr/en/inks/history.php
I got the sepia shade (the only color the store had) but I see they have bistre, which is the traditional tone. A couple quick tests with this and I'm clear now on what materials to use and when:
Basically, the ink provides a stronger, more even coverage that doesn't dry lighter as watercolor does, so it's the optimal choice for compositional studies where you want to lay in simple washes to denote shadow and light, without modulation or detail. For rendering or detailed studies, I still refer the watercolor for the subtlety. You want to combine ink washes with an ink line (pen or brush), but the more delicate watercolor can be laid over pencil.