MIUI == Android + iOS. MIUI is another Android ROM that I love on HTC HD2. It has the look of iPhone but still powered by Android. MIUI is developed by China Android developers and supported by developers from all the world. And it is updated every Friday. Therefore, you can rest assures that you are getting the latest and greatest Android features from them. MIUI for Samsung Galaxy S II is on development stage but most of the functions are working. Do visit MIUI website to find out more and learn all its wonderful features.
MIUI v1.8.5 is the version that I tested on Samsung Galaxy S2. Quadrant score is lower but I do not feel any slowness. Do visit XDA website for latest builds.
Installation guide ~ How to install firmware on Samsung Galaxy S2 with ClockworkMod Recovery?
Download MIUI v1.8.5 CWM ROM here.