I also came here from a Thunderbolt. The rooting and ROM installation process is slightly different than what was done on the TBolt, but if anything its even easier and as with the TBolt there are lots of smart people around to learn from and get help from. One thing you will come to learn is Aroma, which is where a lot of the customization happens. bets advice is to read through the various developer threads to get a Sense (oops, no pun intended) of what ROMs do what, look like what, and have more or less challenges. You will also see threads that will walk through various ways to gain root, unlock the bootloader, and flash ROMs. The good news is there seems like there are way more good ROMs available for the S3 than for the TBolt.
Android devices: Samsung Galaxy S III running BoneStock 3.4 (retired phone: HTC Thunderbolt running InfectedROM NuSense Eternity); HP TouchPad running OaT CM10.1