I have an oldish phone - a Samsung Galaxy S3. I don't feel the need for anything faster or better because I don't use it a ton. I hardly ever call anyone or have anyone call me and it's mostly used for emergency remote access to the forum, to play the odd game and to text the kids to let them know I'm picking them up or they want to be picked up from somewhere (maybe I should sign up as an Uber driver? Does everyone become a taxi driver for their kids?).
Of course it has started showing it's age and after 3 years the battery didn't last so well and it would be a bit sluggish sometimes and get very hot.
That was until I updated it !
No, not with any Over The Air updates that Samsung / Rogers provide because they stopped providing them (I guess they would rather sell you a new phone). Instead I put a custom ROM on it (CyanogenMod 12.x based on Android 5.1.1). Very easy if you follow the instructions. So does it make a difference?
Holy crap, it's like getting a new phone!
All the latest Android UI slickness and it works better than it ever did. Before it would struggle to last a day and I was always turning off WiFi or GPS to save the battery. I've just had it run for 3 days with everything enabled plus it doesn't get hot anymore.
One thing that made a huge difference is removing things like Facebook which is an incredible battery killer. So many apps now are either badly written and / or want to keep themselves running and notifying you of things all the time. Instead Ive stuck to Google / MS provided apps and just a couple of essential games.
I can also change the CPU clock speed now to decide if I want better performance or have a very long battery life.
I've also put the same thing on an old Samsung Galaxy Tab 10.1 and that too is now faster and slicker than it was when it was brand new. Obviously Android is improving and they put a lot of effort into making things smoother.
So, if you have an old phone or tablet I can definitely recommend trying out a custom ROM instead of shelling out more money for an upgrade, especially if you are on 4.x or older.
This is a good place to start:
http://forum.xda-developers.com/
(I found the latest build on a device-specific site, not on the cyanogenmod site)