Had my HTC One for about 3 months now and although its a great phone, I think I'm going to go back to iOS. Not huge reasons for it, just that after spending a day with the wifes iPhone5 yesterday instead of my One it was the little things thats made the difference to me.
iMessage is one. All of my friends and family bar 1 have iPhones so everyone uses iMessage. I got a fair chunk of them to use WhatsApp so they could still IM me, but I get far less messages from them now than i did before. Esp with my folks living back in the UK and me in New Zealand, they forget to use WhatsApp so don't send me half as many whereas they still iMessage my wife a fair bit. And also get no video chats anymore as they can't seem to get Skype working, whereas before we would FaceTime no trouble.
Size. I wanted the bigger screen coming from an iPhone4 and although I love the HTC One screen, there are times when its too big. I like to use the phone one handed when I'm walking thru the city, can't really do this with the One.
Lack of physical home button. This didn't even register on me until a few months after using the One, but so used to pulling my old iPhone out my pocket, spin with one hand, unlock, reply to message/change track etc, put back. Now its a multiple step, multi hand to do anything like this. Sounds silly I know but its something I notice.
Camera. This is the number 1 reason for me ditching the iPhone 4 for the HTC One and now will be a big reason to go back. Its just too hit and miss for me. Indoor or outdoor I can't be sure of getting a decent shot with the One, a large percentage are out of focus still and it tends to hunt for focus a lot when trying to take pics of my little girl running round. My wife's iPhone5, none of these issues, snaps a great shot 9/10 times. Don't get me a wrong, the One camera is good, esp in low light, but for a quick snap its not reliable enough for me.
Battery - I'll miss this about the one. Superb!
Android - Now this was my first Android phone after years with iOS and I really like it. Only things that have been a bummer is that the app consistency isn't there like it was on iOS, something I never thought about with iOS. And I have a large amount of app crashes on the One which I don't remember experiencing before. Not a huge issue but have noticed it. But I really did like the freedom it gave you, especially in changing the default apps etc.
Overall though I think the One is still a brilliant device and Android is certainly a great OS. But for me especially with all my friends/family on iMessage and not willing to move to WhatsApp and also the poorish camera I'll be back to iOS when the 5S comes out here, or may just pickup a second hand 5. Gave it a good try and would certainly recommend the One and Android to anyone.