CyanogenMod is one of the most popular custom ROM for the Android OS. Currently over 500.000 Android user have installed CyanogenMod on their device. The CyanogenMod team supports from for almost any Android device now including the Samsung Galaxy S2.
Nevertheless the first versions seems to be quite stable. Of course it is expected to improve performance as well for later releases. The download links is available in the well known Smartphone developer forum XDA-Developers.
Official CyanogenMod statement:
CyanogenMod is an aftermarket firmware for a number of cell phones based on the open-source Android operating system. It offers features not found in the official Android based firmwares of vendors of these cell phones.