I have a 4S and the only thing I can complain about is the battery. I bought my 4S as soon as it came out around October of 2011.
-It's back is completely shattered, gives it a unique look and doesn't lose functionality. I don't care.
-Lock button barely works, I use assistive touch and a firm push makes the button work, I got used to pressing it down firmly, it doesn't have the clicky feeling anymore it's flush against the body.
-It's got a lot of dents and scratches at all four corners of the body.
-I am happy with the speed (using 7.1) That extra couple of seconds won't make me gain anything. Plus it's got all of the major functionalities of existing iPhones. Previously if you had the 3G you didn't have video, if you had the 3GS you didn't have the front facing camera etc. Extra features of the 5S such as the fingerprint and burst mode or slow-mo is trivial if you ask me.
Despite all of this only thing I am upset about is the battery. I have to charge it at least 3 times a day. 2 with light use. I don't game, I only check social media and chat on WhatsApp, occasional snapchat etc. So I wouldn't say it's heavy or moderate use.
Not only the battery life is short, it also has weird habits. Even if my phone is at ~40%, taking a photo usually makes the phone turn off and give a low battery warning, not to turn on again for at least 30 minutes. If the weather is cold any my phone got cold as well, it just turns off, again giving the low battery warning. I have followed all the battery saving tips etc but it doesn't help. I am destined to walk around with a charging cable in my jacket's pocket and an additional battery pack iPhone case. I don't know what to do when the weather gets better and I don't have to carry a jacket anymore, I can barely find space for my keys.
So if you ask me, Apple shouldn't even bother making the iPhone 6 thinner. they should shrink the components as much as they can still and make the body as thick as the 4S if necessary, then fill the rest with batteries. I want an iPhone that can get through 24 hours with extreme heavy use under all conditions. Only then it will become the perfect phone. It's almost as if I am an electric car user with range anxiety.
I'm sure the vast majority of iPhone users can relate with me on this.