Well i have the Mophie Juice pack plus, I think the build quality is very good and i have not had any problems with the usb charging port. I think the problems where they break has been on the Juice pack air, the sides of the juice pack air is plastic where the Juice pack plus is more like rubber. I also like the juice pack plus because it has buttons for the volume, and sleep wake buttons are built in and makes it very nice and natural to use, Just like using the phone with no case at all. The others that i have seen have deep cut outs that make it difficult to push the buttons. Also the Mophie directs the sound to the front instead of the bottom and makes the sound alot better, kinda like its own little speaker box. Sure the price maybe more then the others but i think the quality and ease of use is well worth it. I would def buy one again if something ever happened to this one. I get 3 to 4 days of use between the phone and juice pack plus before i have to plug it in to recharge. I could get by with out having a external battery pack at all, but on those days when i am a very heavy user it sure is nice having that extra juice for when you need it.
as for added bulk at 1st you notice it, but after a while its nothing. Doesn't bother me at all, In fact if i take my iphone out of the mophie i feel like i am holding an egg and don't want to break it. The mophie makes if feel alot nicer to handle and alot more rugged. I think most people would be very happy with the mophie. Spend your money wisely and on something that you would like instead of buying something cheaper and having difficulty in using.