I do pull-ups on a consistent basis. I mix it with underhand narrow grip, overhand wide grip, and v-grip. Of course, your gym will need a v-grip pull-up bar to do that.
They improve lats, biceps and forearms.
Just like anything else, you need to mix up the resistance. If your gym has them, use the weight belt to add resistance so that your reps per set is between 1 to 6 on the heavy days.
When doing pull-ups, extend the arms fully when you are at the bottom of the pull-ups. This allows you to work the long head of the bicep. If you don't, then you won't work the long head as much. For those who subscribe to the constant resistance school of thought, you can keep your elbow flexed with constant bicep flex and not bottoming out, but this mainly works the short head of the bicep. If you want mass, fully extend in order to do the most work per rep.
Cheaters don't get the V, if you know what I mean.
The V-shaped back.