I bought two bags (messenger and sling).  So far neither adequately charges our devices.  My expectation was we would store our Nook, Galaxy Tab, Samsung Seek phone and incidentals like ear buds, owners manuals, cleaning wipes, you get it.

The built in tails are a very tight fit on devices for which they are designed. . . a concern of mine.  It does not charge a tablet.  I mean, I used USB port and it does not charge a tablet.  The indicater on the tablet shows the charging lightening bolt with a red "X" .  It does charge the cell phone.

The Sling Bag was DOA.  The company sent a replacement but I had to pay shipping to return a bag that never worked.

We find the cushioning built into the bag interferes with the amount of stuff one can pack.  Do not expect it to function as a carryon or backpack.

I do find the Sling Bag to be very comfortable to wear.  I like the cable and cable length that charges the battery pack.

Even though I planned for my spouse to keep all mobile stuff in the bag I bought for him, he does not.  He feels it does not hold enough and is larger than he wants it to be.  Basically he is complaining about the padding that means the bag takes a shape and remains in that shape.

Too much $$$ for how we use it. . . as  cell phone charger.  Hope it works for all Apple users.