Jellybean on my Galaxy Nexus, impressions:
* The smoothness and responsiveness improvements make a real difference. Absolutely awesome.
* I like that the "Recent Apps" list now expands from the thumbnail into the app (you'll understand if you use it). A small thing, but shows attention to detail.
* Google Now... is OK so far. It predicted my need to drive home from work on Friday and alerted me to traffic conditions, although it's also done the same thing for going to work on the weekend (which I don't). Weather is working, but not much else.
* Addresses in the Calendar app don't easily link to Google Maps (still). Wish they'd add this -- it's in the web versions.
* New rich notifications are busy, but powerful and useful.
* New G+ app is better, I really didn't like the photo-text stream from the previous version.
* Voice recognition is much better than it was. Faster and more accurate. I might now actually use it from time to time!
* Battery life seems better when running WiFi, bluetooth, GPS etc. etc.
This is coming from a custom ICS ROM with better than stock smoothness and battery life already, so users of stock ICS will notice the improvements even more.