I have to give it to Mophie. I use my phone a lot while I'm out and it always tends to be stuff like YouTube, constant Twitter use, AppStore (Searching and download), etc. My batter life goes down quite a bit, but when I've needed it most, it's come through for me.
Me and my brother drove me sister up to a shooting competition up in Oak Harbor, WA. From where we are (Oregon City, OR) it's about a 6 hour drive (Including stops). While we were up there we stayed the night, but the day of the competition was also the day we had to drive back home (I didn't know this at the time). So they finish shooting, get scores, do awards and we get in the car and my brother asks me to use my phone as a GPS to go home. I was at 20% battery at that time so I turned on the Mophie pack and voila within about 45 minutes my phone had a good charge in it.
The Mophie pack is a great thing to have around regardless. Even if you prefer to have a different case on, the Mophie is a life saver.