From the off, the rom seems noticeably faster and smoother to me.
1. The new app transitions are fantastic. They fade in and out, which is quite slick.
2. The keyboard has ZERO lag, I didn't think it had any lag when I was running AOKP, but running the new JB rom, it's a lot faster again!
3. Home screen/App drawer transitions are a lot snappier. It is now on par with the transitions of an iPhone 4s (Compared with my friends earlier) No lag in the app drawer, even with all of my many apps installed.
4. Last but not least, scrolling long lists is buttery smooth for me
Everything just seems so slick at the moment.
On a side note, some subtle changes, like when choosing default action, or long pressing on home screen look a lot nicer. The lock screen looks way cooler. And of course google now seems to be working great. (Aside from having to tell it to use English US, because its not fully functional in the UK)
Hope this helps anyone who was maybe sitting on the fence, I have been running AOKP since getting my nexus, Think is was build 11 or 12 I first had. I am now running Jelly-Belly by Team Inferno and have not even though about returning to AOKP. I will gladly return to AOKP when they release a stable JB update (which will likely be in August some time (JB Source to be released in mid July))