In the video below you can see for yourself that Android 4.4 is actually running very smoothly on Galaxy S2. But this is not surprising, since the smartphone installed custom firmware CyanogenMod 11, which is based on Android 4.4.
While there are still some minor bugs in the firmware in the video is also shown. Still fragile works video recording, but it is only a matter of time and soon firmware upgrade will work flawlessly.
If you are interested to install this on their own firmware on your Galaxy S2, you can look at this link on the XDA forum. There you will find everything you need for flashing: instruction itself, and the necessary files. But by itself add that you will be solely responsible for all actions and consequences.
Anyway, time to watch the video: