For T-Mobile SGH-T989 S2 users, you are in luck as this version is actually better than the rest with almost everything working except incoming calls, 1080P video recording, and must keep wifi turned on at boot (otherwise will reboot on sleep).
Overall, it’s still WIP (Work In Progress) and in a few weeks we should have all the kinks worked out. In the meanwhile, if you must try it today, just make a backup ROM, try Lollipop, then restore your ROM.
For installation on GT-i9100 or SGH-i777, please follow How to Install Android 4.4 KitKat ROM on Galaxy S2!
IF YOU HAVE CWM RECOVERY OLDER THAN VERSION 220.127.116.11, UPGRADE your CWM Recovery to latest version by downloading the zip and installing with your CWM recovery OTHERWISE ROM WILL NOT INSTALL!