The GS3 was also my first all-in ROMing device. I started by putting official leaks on my Bionic, which did not require any serious risk. Once I knew I was getting the GS3, I just started reading, a lot. I have every tutorial and guide on XDA bookmarked, for just about any situation. In every guide, I read just about every post to see other people's issues, tips and tricks. There can be a lot of info gained after the first 2 posts. Definitely make sure you back up your IMEI number before you do ANYTHING! I also recommend having insurance on the phone. I personally do not much mess with the custom kernels, but have flashed them. Most flashing is really as easy as just clicking a couple of things and bam, it's done. The important part is making sure you are all set up to do it, and have READ THE DIRECTIONS multiple times.
The big advantage, besides being interesting, has been battery life and Google Wallet. I have run 3-4 AOSP ROMs, and I don't know how many TW ROMs. I reallllly like AOSP better, but keep coming back to CleanROM with Wasabi's Themes. I have yet to find a "perfect" AOSP ROM. I probably liked Baked Bean the best, but still had some nagging problems.