I've always incorporated pull-ups into my back workouts and will honestly admit it took some time to master them and actually feel my back doing the work, but the time spent learning them was well worth it. The trick is to visualize the use of the back muscles (Lats, rhomboids, traps, etc.) doing all the work- this is called the mind-muscle connection, and to think of your arms simply as "hooks" holding onto the bar.
This visualization will eliminate the biceps from doing all the work and you will be able to pump out more reps. As you fatigue, your biceps will kick in and begin to help a bit- so they will get some work but the back muscles should be the main focus. The core also stabilizes you in the pull-up, so the abs get involved to a degree to help get you up and down.
I started my back workout last Sunday with a few sets of overhand pullups, chin-ups (underhand) and Cliffhangers which are a Navy Seal variation on the pull-up.
By Monday morning, my ENTIRE back and biceps- not to mention shoulders and abs were more sore than I have gotten them in a long time. Yes, I love being sore....it feels great and just knowing the soreness was caused by all of those pullups made it that much more significant.
My clients all get to learn pull-ups at a certain point in our training and I don't allow them to substitute the exercise for anything else, because there is nothing to replace this challenging move. My motto is hardcore and old-school and only until you master the basics can you fully understand the mind-muscle connection and move on to easier exercises. If your back workout is growing stale, head over to the pull-up bar or the assisted machine and discover a whole new way to build a strong body.