A couple of hours with Polish JB, and I must say the following are the things I have noticed the most:
(A very non-technical guide to Polish JB for those who haven't tried it yet).
1. Its smooth as silk (buttery even.... :-P).
2. Its feels nice and fast and responsive compared to stock XT ICS.
3. Despite point 2. it is not as fast and responsive as CM10 or Supernexus.
4. Some nice little features added in by Samsung over and above ICS, it's not just Project Butter, and a new name, its a new 'ROM' (in the sense that the developers have released more new features than AOSP 4.1 alone).
5. Yay for Samsung camera app.
6. Will give this a chance for a while, but might ultimately find myself back on CM10.