I've also tried out MIUI on my Galaxy S and was very happy with it.
@DeliciousRaspberryCake The factory ROMs aren't that bad, which is why I went back to Gingerbread after a few months of MIUI. It was good, but not quite as good as the Samsung ROM.
Interestingly, on phones like the Galaxy SII, Samsung support custom ROMs (maybe not "support", but at least officially condone it). It makes sense - they provide something that they think is the best option, but are happy for users to try other options. HTC for instance, do not - the Sensation is locked down to their bootloader.