1stly, After getting JB update from OTA, it's recommended to do a factory wipe in stock recovery (This is what Samsung CS told me). I tried and it works (at the expense of having all my data n apps gone).
2nd, Like @forest1971 says, there's many good custom Roms And you can choose per your liking. I strongly oppose going back to ICS as of the brick bug threat. If you really want to downgrade, might as well go for GB.
3rd, WHAT is the point of our good Dev's spending their precious time and cracking their heads to come up with a custom rom IF they are not BETTER than Stock roms?