I can think of times this would be helpful, long road trips where everyone might be fighting over one charger, long flights, camping. I just can't see myself having it permanently on my phone. Here are some other options I found quick on Amazon just in case people are interested. For me personally, I think I'm going to get a 2 pack of standard batteries that come with their own external charger. I can keep those in my backpack (or one in my pocket) and between the regular one and backup I can't see myself needing more.

• Slightly more manageable 5200mAh ($25)
  http://www.amazon.com/Hyperion-Samsung-Extended-Compatible-Warranty/dp/B00BCK2G40/ref=sr_1_6?s=electronics&ie=UTF8&qid=1371040849&sr=1-6&keywords=galaxy+s4+battery
• A case for the Hyperion 5200 battery that looks pretty sick http://www.amazon.com/Hyperion-Samsung-Extended-HoneyComb-Packaging/dp/B00CXO09DY/ref=pd_sim_e_2
• Set of two standard size, supports NFC and Wallet, comes with a charger, looks promising to me ($25) http://www.amazon.com/Hyperion-Samsung-Batteries-Compatible-Warranty/dp/B00BCJWYMU/ref=sr_1_8?s=electronics&ie=UTF8&qid=1371040849&sr=1-8&keywords=galaxy+s4+battery
• 1 Hyperion standard size, with docking cradle that charges the extra battery (what sized cases fit?) ($30) http://www.amazon.com/Hyperion-Charging-Includes-Compatible-Warranty/dp/B00CP8EK1U/ref=pd_sim_e_3

Does anyone have any experience with these Hyperion brands? They all come with an 18 month warranty which sounds pretty sweet.