I upgraded from an S4, and I like it. The screen is the major selling point for me, as is the fact that it's water proof. So far it seems to be faster than the S4, and I can't wait until we get AOSP ROMs on it. I haven't played around much with the camera yet, but a potentially better camera (from the reviews) was another reason I upgraded.
I don't agree with the back cover comment, to me the phone is like a bar of soap and slips out of my hand way too easily. I should be getting a Ringke slim cover for it today that should help. FWIW I have the white version, I read somewhere that it was more slippery than the black version but I don't know if that's true.