4.1.1 Jelly Bean has FINALLY officially been pushed to all 4 major carriers for the U.S. GS3.
I can't really speak on the other variants updates. I don't know what the other carriers like to do but I know that Verizon Wireless likes to put in their tweaks and they actually have to approve the updates that come from Google, Samsung or any Android manufacturer. Yes, Verizon actually had to approve Jelly Bean that came out on their Galaxy Nexus. That's a Nexus!! No more Google phones for Verizon I would say. They support more GSM carriers anyways.
ANYWAYS, the JB update brought some cool new things. Some speed tweaks I see. But I also experience some things I'm not too fond about. Default weather widget won't update city if the GPS is not on. Before, it didn't matter I could be on any data service and it'll update when I refreshed it. Not a big deal, I use beautiful widgets. Thing I hate the most about the update is JB has been giving me HORRIBLE battery life. Suppose to be a refined OS, way to fail Android. I even heard the Nexus 4 battery life isn't all that either and that phone is HSPA+ with a quad core processor for better management.