Much much better than the Mophie Juice Pack Air. I bought this pack to replace an 18month old Juice Pack Air, and what an improvement.
My old Juice Pack Air only charged my phone to 65% charge from a 80% depleted iPhone 4S battery.The new Juice Pack Plus charges the phone to 100% from 88% depletion, vast improvement. (The comparison was done when not using the phones apps or calls to try and ensure like for like use.)
True It is 50% more expensive than the Juice Pack Air by £30 but it provides more than 100% extra power.
This pack has also removed the continuous blinking lights from the power indictors so that they only stay on for 30 seconds to let you know the battery is being charged.
If you use your phone a lot when you're out and about and not near a charger you will find this battery pack possibly invaluable.