My first S4 broke while in my pocket on 5/6, I wasn't doing anything out of the ordinary, was just doing things around the house, when I pulled it from my pocket there was a crack from the upper right corner down the middle of the left side. The AMOLED display under the glass is what broke, device was unusable, nothing showed on the screen except some colors on top left above the crack.
After fighting with AT&T and Asurion the entire next day, AT&T agreed to give me an early upgrade. So I bought a new one on 5/7.
I received my new S4 on 5/10, then this morning, 5/15, while at work I was squatted down, texting, and dropped it from around knee level on carpet with padding over concrete and it cracked the AMOLED display, this time in the upper right corner, with 4 cracks from the side to about an inch into the display.
This S4 had a Spigen Ultra Thin Air case on it, as well as Spigen's Glas.t Slim screen protector.
I don't expect manufacturers to make phones that are indestructible, but this seems ridiculous to me.
I was a long time Apple user before I bought the S4, I don't think I will ever go back, but I'm regretting going with Samsung now. I've spent over $900 between both devices and now they are both in the mail (as well as almost $200 in accessories that are worthless now), for Samsung to evaluate, so for the next two weeks I'm stuck with using an old S2 that likes to reboot every 15 minutes.
I've read a few posts on here about people breaking the screen under the glass like I did. So I'd be careful if you own one.