I tried to "pack" my shoulders in more today and was able to do sets of 4-5 wide grip pull ups. I feel it a little more in my back now too, but still more in the arms/shoulders. It does seem to hit the posterior deltoids pretty hard which is nice.
Wide grip is the way to go! Its a compound work out (well so is the narrow grip... sort of), so you are working a lot more of the lil stabilizer muscles. You will still work your biceps but the focus will be on your lats and mid traps.
Narrow grip will work your lats and a bit of your traps but the focus is on your biceps.
Always hang from the bar after you are done... build up that forearm strength!