I've had an iPhone 4 for 2 years, then iPhone 5 for 2 years. Always use a case and put it in my front left pocket. They both looked like new when I sold them.
Got the 4.7" 6 on launch day, and immediately put it in an Apple silicone case.
When the bending stories came out, I thought it was just a few people who had mistreated their phones, but just to check, I took mine out of the case and put it on a flat surface. It was perfectly flat, this was last Thursday I think.
I tried it again last night, and it's very slightly bent, not towards the rear but towards the screen. (I always put it screen side out in my pocket) You can't tell by looking at it, but if I put the screen on a flat surface, I can push the middle of the phone down, less than a millimeter, but still noticeable.
For now it's not enough to bother getting it exchanged, but I'm OCD with my phones and it still bent a little after only a week. It could be a defect with the aluminum in a certain batch, but if they're all this way, Apple will be exchanging alot of phones in the future.