I'm using the Mophie Air now. I agree it does add the extra weight, height and length to the phone, but surprisingly the extra does make the phone easier to hold and feels more manageable if that's the correct word. Not to mention the additional 1700mAH backup power (at the flick of a switch and without carrying separate power bank).

If you're getting Mophie, my advice is to get the Air and not Helium. I've no problems charging my phone from 20% to full (with 1 bar of power to spare).