When I purchased my iPhones, the first thing that I found was that the battery life ended around 10:30 AM.  This was NOT good.  So I began my search for a batttery backup assist.  I looked on the web at all of the then advertised external battery cases and external batteries and found the IMP500 at then IMAX, which was since bought out by New Trent.  I chose it because it had a 5,000ma capacity and was about the same size as my phone, which made it more convenient to carry arround.  It came with attachments that allowed me to also connect my BlackBerry phones as well.  I really like the retractable cord).  The only problem that I had with it was that it would periodically get turned on in my pocket, but they very quickly modified the unit with an automatic shutdown to prevent that from happening.  I have been very satisfied with my battery pack since its purchase when they first came out with them.  My co-workers, friends, and family members have used it on several ocasions when there batteries died, and I still had backup for mine when needed because it allowed me to charge up 4 1/2 to 5 phones before I needed to recharge it.  I have since purchased others, but I still like this one the best.  I don't have the version that automatically shuts off because I purshased mine before that time, but it doesn't create a problem for me, but if you purchase one today, it will have that functionallity.  I take mine with me on horseback rides out in the country over the weekends and have plenty of extra power when needed.  I have considered purchasing another one because others keep borrowing mine to charge up their phones.  Great product!