Having received my S5, I can say the differences between the S4 and S5 camera aren't as pronounced as I would have hoped. There is a difference in the quality of the final shot in favour of the S5, but the process of taking the photos, in terms of shot speed and auto focus speed, isn't that different, especially in low light conditions. Actually, with the new low light 'hold still' shot mode of the S5, it's slower, although the final result is much better.
I'm actually debating returning my S5 and returning to my old S4 contract which only had 12 months to run.
Previous phones - Philips Diga, Nokia 6110, Nokia 3210, Nokia 7110, Nokia 8890, Nokia 9110i, Nokia 8310, Ericsson T28i, Nokia 6310, SE T68i, SE T610, Motorola V600, Nokia 6230, Samsung D500, SE K750, Motorola V3, Samsung D600, Nokia N73, HTC Artemis, LG KU990, Nokia N95 8gb, HTC Leo, Samsung Galaxy SII.