Long time iPhone 4 user (3 years) who just got an S4 because I lost my iPhone (or it was stolen) the other night. I just wanted to say that SmartStay is actually one of my favorite features. It allows me to set the screen timeout time to 15 seconds to save battery life (which is pretty average on S4, btw) and read without worrying about the screen shutting off. It also works at night while I'm in bed with the lights off, I suppose the light from the screen might be enough, or it also has some accelerometer input or something.
Granted, this feature would not be as useful on the iPhone as it has a smaller screen (not knocking iPhone, one of the things I dislike about the S4 is that it's too big.) and it requires one to scroll more frequently. But why would I need the screen on when I'm not looking at it? It just wastes battery life. I think this is something that would be great for the iPhone.
As for it killing the battery life, I haven't had the phone long enough to tell whether it does or not. I'm sure it drains some of the battery life, especially if you're reading quite a bit and have the screen timeout set to 15 seconds like I do.
Hope this helped!