I wanted to check out the S4 and so I went to the Samsung store yesterday. There were three S4s on display so I picked up one and started trying it. First impression was good and bad. While I loved the screen and the new home button (with the side capacitive buttons), I was very disappointed with the shabby build of the phone. The edges were made to look like metal (while they were plastic) and felt really cheap. Also, the weight of the device was disturbing. Yes, disturbing. It was so light I couldn't even feel that I was holding a phone in my hand. But your brother might just like this.
On the software part, I tried to open the camera from the main screen and the phone conked off. The Samsung guy told me that it had started updating (wtf so randomly?!) and gave me the second piece on display. So I again tried to open the camera app. While the camera app opened, the viewfinder never showed anything - it was plain black. So then I checked out the third device (*whew*) where the camera actually turned on. Although the picture quality was decent, there were lot of gimmicky features which the rep tried to sell to me but they were all really pointless (why would you want to spoil a perfectly good picture of someone with an tacky inset picture of yourself?) Other than the camera, the screen and the eye gestures didn't really seem to work well as the rep had to continuously switch the 'Smart eye' and 'Smart gesture' mode on and off.
Ultimately its the reliability and the polished experience that WP offers which wins my heart! I hope I helped you on this