I have had the Juice Pack on my S3 for a long time now.
The Good:
Effectively doubles the battery life.
Always with you.
Case is pretty durable and has sustained a few pretty bad falls.
Has an On/Off button so you can choose whether to drain down your phone battery or the battery back first.
Charges the pack/phone simultaneously when phone is charging.
The Bad:
It sustained so many falls because the outside of it is a very, very slick glossy plastic that makes the phone like holding onto an ice cube.
Doubles the size of your phone.
The top part is pretty flimsy and can crack in half easily if you remove it the wrong way.
Makes the phone longer as well as thicker.
The part that wraps around the bezel of your phone and hugs the back is hard plastic and has scratched my phone.
It's expensive. Too expensive for what it is.
Looking at the S4 Juice Pack, I can tell you that none of these things has changed, so take it for what it's worth.