You can now update Galaxy S2 to Android 4.4 KitKat based OmniROM nightly firmware. OmniROM is based on AOSP Android 4.4 KitKat packing a lot of features. Many of them are also from the stock KitKat:

Flippable quick settingsDSP Manager enhancementsNon-obtrusive incoming call notificationMulti-window and multi workspace modesPer-app permissions and many more

Follow this guide to update your AT&T Samsung Galaxy S2 i777 to OmniROM Android 4.4 KitKat Nightly.

Prerequisitesbefore youupdate Galaxy S2 to Android 4.4 KitKatbased OmniROM custom firmware:

Create a backup of all important data in your Galaxy device.Download and install USB drivers forSamsung Galaxy S2in your computer.Enable USB debugging option by pressingMenu>Settings>Applications. Navigate and tap onDeveloper Optionsto ensure thatUSB Debuggingis enabled.Make sure that recovery tool like ClockworkMod recovery is installed.Ensure that your AT&T Samsung Galaxy S2 carries more than 80% of charge.

Guide toupdate Galaxy S2 to Android 4.4 KitKatbased OmniROM Nightlies custom firmware:

Files required:download the Android 4.4 KitKatbased OmniROM custom firmware for AT&T Samsung Galaxy S2 andGoogle Apps. Save both files on PC.

Connect your SamsungGalaxy S2 i777 to your computer using USB cable and copy the downloaded files to the SD card of your Galaxy smartphone.Disconnect the device from PC and turn your GalaxyS2 off.Now boot in to custom recovery mode: press and holdVolume Up, Downand thePowerbuttons simultaneously until Samsung logo shows up. Leave the keys for half a second and hold again. UseVolumekeys to scroll andPowerbutton to select -alternatively you can useQuick Boot appto reboot your Android device in recovery mode.Now perform Nandroid backup of your current ROM: selectBackup and Restore>Backup. Go back to main recovery menu after backup process is done -very important step to restore back to current OS.Perform data wiping tasks: selectwipe data/factory reset>Yes. It will take a few minutes; once done, selectwipe cache partition.Next tap onAdvance>wipe Dalvik Cacheto wipe Dalvik cache data too.Selectinstall zip from sdcardfollowed bychoosezip from sdcard. Locate the Android 4.4 Kit Kat based OmniROM nightly file copied earlier and select it. Tap onyesto confirm installation and wait for the Android 4.4 ROM installation to be completed.Once installation process is finished successfully, repeat the above step but chooseGAppsfile instead to install the Google apps package on your AT&T SGS2 i777.Return back to main recovery menu now and selectreboot system nowto boot up into the latest Android 4.4 KitKat OS. First boot may take a good few minutes.