I have a 4S. I had it on 7.1.2 for the longest time because I was really hesitant to update it after reading the hundreds of messages on these forums about the 4S and its problems with iOS 8 in general.
However, after doing a lot of thinking and also seeing that numerous apps no longer support iOS 7 in their updates, I decided to take the risk last week and upgrade to 8.2. I did the update in iTunes, restoring my backup afterwards, and I really didn't like how it was. I had a really slow, overheating phone in my hands, and in terms of resignation, I decided to try and do it again.
The second time, I did not do any sort of update per se or a backup restore of the device. I saved the files I didn't want to lose on my computer before doing it, and I did a totally fresh install of 8.2 via iTunes, (clicking restore, not update). After that, I have to say that I regretted not doing it sooner. My phone has no problems whatsoever, not even lag compared to 7.1.2. Same battery life, (not great of course, but not worse than before, that's my point), no over-heating, and no WiFi issues.
Yesterday I followed the exact same procedure for iOS 8.3, and same results... a fully functional phone, just as it was on 7.1.2. Not better, not worse, but with all the iOS 8.3 features.
However, you should also keep in mind that I always tweak the phone's settings A LOT to make it work better. I did it on 7.X, and I'm doing it now on 8.X. And comparing my phone's performance to some friend's 4S that don't do those things, the difference is really noticeable.
Some people might not like the things I configure, saying I'm "underusing" the phone, but for example: I turn off background refresh completely; I pretty much disable Location for almost everything; I only have Notifications set for a few apps; I set my email to fetch hourly, not push; I turn on Reduce Motion; I disable transparency (MAJOR performance and battery life improvement doing so); I removed all widgets from the notification center; I disabled all vibration alerts when the phone isn't silent; and I keep my screen brightness at about 30%.
As I said, some people say that doing all those things is underusing the phone, and maybe they're right. But at the end of the day it's my phone, and I want a perfectly functional phone that does what I want, when I want, and how I want it. If having transparencies, location tracking all the time, and dozens of apps sending me notifications will not let me have my phone working as I want it, then I clearly don't need those things.
If I had a 6 or 6 plus, I probably wouldn't bother with all those settings, since I know the phone can handle it easily, but I'm aware my 4s can't do so, so it's up to me to look for a solution or live complaining about my phone being slow and unresponsive.
Sorry for the long post! I hope maybe some of the things I said can help you!
