After using it for several days now I can say I have experienced no lag at all. I feel like the phone is finally performing up to it's real potential on this JB update. Project Butter seems to have worked for me.
Lag was never an issue on this phone - pretty much ever. On GB and ICS the phone has always been lag free however I'm not sure what others install on their devices.
There is a bug in this update, and it's a fairly big one for me. You basically cannot use any third party keyboards as the phone randomly resets the input method back to Samsung's stock keyboard, which wouldn't be an issue of the damn buttons weren't so super small on it.
Set it to SwiftKey 4, and it almost always resets it back to Samsung Keyboard when you reboot. Then randomly during the day even without a reboot it will reset the default keyboard. Seems like a fairly trivial thing to let pass through QA, IMO (considering the phone comes with 2 keyboards preloaded...).
Battery Life on WiFi seems to be getting a little better... Battery Life on HSPA+ and especially LTE is worse than it was on GB and ICS, by a noticeable margin. Will see if that improves later...
I'm not sure I like the new Notification Shade. You can barely see anything because of all the bull**** they have inserted into there...There's Date/Time, then Quick Toggles, then Brightness (seriously?), and if you're on WiFi you literally have always an Ongoing notification telling you you're connected and what network you're connected to. Just by default half of the notification screen is unusable and mostly because of the useless WiFi Notification and Useless Brightness toggle in there. If you get any real notifications, you now have to scroll to see them when before you simply had to pull down the shade and view it.
Google Now has proven to be utterly worthless so far, but that may be because I don't have much going to my Gmail account and I certainly don't have a Google+ Account/Profile.