One of the most useful gifts Kathleen has given to me since we got married is the Mophie Juice Pack Plus rechargeable battery case for my iPhone 4S. I really could not get through an entire day meeting with clients in New York City without it.
I generally start the day using my iPhone without a case. I take good care of my phone and I like the small form factor that made the iPhone famous.
When my battery level hits 20 percent for the first time, I take my Mophie out of my backpack and snap the case on to the phone. This increases the size and weight of the total iPhone package, but not that significantly.
The Juice Pack's design allows it to act as a booster for the iPhone's internal battery. This means that the iPhone can show up to a 100% charge after it's been connected to the Juice Pack for a while. But generally, the Juice Pack will boost the iPhone's battery reading to between 60 and 100%, at which point the Juice Pack's battery will be exhausted, and I can take the iPhone out of the case again.
This solution allows me to spend a whole day in New York City meeting with my customers, business prospects, and partners, and generally never have to seek out a wall socket. It can also get me through the day when I need to participate in a long conference call in the middle of the day, but I can't get home until dinner time.
For iPhone 4S users who are not as focused on business as I am, the value of the Mophie Juice Pack is that it can get you through a wedding, a trip to an amusement park, or a child's sporting event with the confidence that your iPhone won't run out of power before you get home.
If you are serious about using an iPhone 4S for personal productivity or for the sheer enjoyment of the experience, you should really consider adding a Mophie Juice Pack to your arsenal.
Update: If you have an iPhone 5, you may be interested in David Chartier's article, A review and comparison of the Juice Pack Helium and Air for iPhone 5, which gives good recommendations for that phone.