Because Android is open there are many developers who make ROMs for popular phones. Because the SIII is proving to be so popular, you can bet there will be a slew of well written ROMs available. Most ROMs out there are basically vanilla android with certain, low impact enhancements by the developers. CyanogenMod is one very popular group, but for some devices, the most popular of ROMs have come from individuals.
I am confident that it won't take long at all to have many options available to SIIIs on all carriers. The process for doing this ranges by device, but is more often than not something that anyone who can follow instructions on a web page can do without much trouble. Of course there is always risk, voided warranties, etc.
All that in mind, most people here haven't yet played with TouchWiz 4. Maybe some of us won't mind it, it seems there are at least a few nice features it brings. Maybe we will like the features but hate the launcher (easy fix there, as you mentioned). Maybe we will all hate it even more than some think. Who knows.