I got mine from Amazon Tuesday. I charged it and brought it with me last night because I felt I would need it, but alas, the stock battery got through the night just fine. I'll install it in the phone tonight just to make sure it works, but really the battery life with the S4 seems just fine to me. There will definitely be times though where I know I will need it.
Yeah I was in the same boat yesterday. Tried to use my phone as much as possible and still was at 60% at 11pm, this battery is really great. However, it will be nice to know I have a whole other backup just in case.
Extra battery FTW today, forgot to charge my phone last night. Even thought I am 1d 0h 22m on battery and still have 43% left, nice to know I have a backup in my backpack.