I received an old Samsung Galaxy SII (GT-I9100) from a friend of mine who said it's no use for him, after he purchased a new Nexus 5. He complained for a long time that the phone is slow and unusable.
Well, this was a pure win for me. I got the phone and the installation of Cyanogenmod mode was just a breeze. It took less then 20 minutes, and I can definitely say a not technical user can do it. Bravo for the Cyanogenmod developers!
The process is very well documented, but unfortunately the process is very well Windows biased. I have an old laptop lying around for just these cases.
Now I have a very well working Smartphone with the Android 4.3.1 available, and I must say this is way better then my previous Sony Xperia E10 which I previously used. This phone is much faster and has support for faster Internet browsing.
I can really recommend people to stop purchasing new Smartphones for ridiculous prices. Just go and get a used Smartphone supported by Cyanogenmod and you got the latest and the best of Smartphone OS.
Besides benefiting yourself with saving money, this has a positive influence on the environment, since you reduce the need for getting new raw materials needed for making Smartphones.
One more benefit of using this Galaxy SII is that you can run Replicant on it. Using Replicant is something you should really consider if you are concerned about privacy! Replicant developers have found that this phone has backdoor introduced by Samsung!