Call it a hunch, but recently i felt the 2 year old galaxy s3 was getting really slow and instead of having to buy I new phone i did what i did with my previous htc G2, namely rooting it and installing cyanogenmod on it. And now, a month ago or so, Samsung announced the GS3 would not get kit kat because it didn't have enough ram, which is funny since Google focused on reduced the required system specs in the 4.4 version, so kitkat would actually take up less space. But Samsung has it's usual suite of bloatware which of course takes it over the limit. Although I do miss some functionality in the now rooting GS3, like just saying "click" og "smile" and the camera app would take a picture, and swiping your palm over the screen would take a screenshot, but then again some thinks really started to tick me off. Like when the 4.3 version came out I was expecting the lockscreen widget and quick camera access would be present, and it was until you activated the pin code/pattern lock security, which pretty much everyone should, but that disable the functions. The notification area also was hampered by samsung engineers since the turn off blue and wifi buttons took up a lot of space, so jumping into known territory with installing a custom rom on it seem to be the last resort. And I have to admit, the cyanogenmod guy have really done a great job. After the installing process, pretty much all of the samsung mods because obselete because the 4.4 version does all these things natively. The install process did take some patience but not in the areas I expected. But the biggest hurdle was to get the install process going and the fix was to use the original samsung usb cable and using the usb port in the back of your pc. Tried the laptop but the install process just kept displaying "could not connect to device".
Anyways a couple of pages helped me a lot since the install process tells you to just go to http://get.cm and install, but that's where I kept getting the "device not found" error.
I went to http://techverse.net/root-samsung-galaxy-s3-android-4-3-jellybean/ and rooted the phone following the instructions using the samsung cable and the correct usb port. This is of course presuming you have the 4.3 version from samsung on it.
When that was done, going to http://beta.download.cyanogenmod.org/install and following the instructions finally connected to the device and it pretty much went without a hitch since from there so I was one happy camper :). It's a completely different world all of a sudden, everything just runs fast, keyboard doesn't take 5 secs. to appear and the stock experience just makes everything all more fluid and butter smooth. The battery life wasn't much different though but everything just ran soooo much better and I can't recommend it enough for other people thinking about taking the cyanogenmod jump.
P.s. I helped a buddy of mine do the same and when it started to install and we left it for a while, the screensaver kicked in and apparently stopped the install and turning the phone off and on got the phone into endless bootloop. We fixed it with doing a wipe dalvik cache using power, home, volume up to get into the fastboot mode . power, home and volume down gets you into the download mode where the cyanogenmod install will detect it ready to install. Wiping the cache though does require you to install the google apps yourself where the cyanogenmod installer allerede does the for you, unless of course the install process gets interupted.
Disclaimer: I take no responsibility for loss of software, hardware, info or hair when following this article. I just wanted to share my expiriences with the latest modding adventure.