I've been using Jelly Bean for a few days now and I have a few scattered thoughts on the changes.
There are significant improvements, but the difference is nothing like the jump from 2.3 to 4.0 (which should be expected from a decimal update).
The effects of project butter are really noticeable. Everything feels much smoother and faster, especially loading and screen transitions. It will be interesting to see how it performs on a Nexus S.
The expanded notifications are useful, and make the notification system much easier to use. I suspect I'll see even more benefit as app makers adjust their apps to offer rich notifications. I really, really hope that includes adding an "archive" button to Gmail notifications.
I really like where they're going with the UI, but I feel like in some ways the UI design is less consistent than it was in ICS. Things like the new notification shade and Google Now don't quite match the older parts of the OS. I'm sure that will be fixed through future updates though.
Google Now is pretty cool. A lot of the stuff could be done before by Voice Actions, but this setup seems more functional and more usable. Now identified my most common locations pretty quickly, and now provides traffic updates for those places when it thinks I'm likely to go there. It also pulls information from my Google searches on the PC and uses that to identify my interests. If you're the kind who's worried about Google knowing too much about you, you won't like this, but I think it's a really neat way to aggregate information in order to make my phone work better for me.
I'd like to see more integration of Now with non-Google Android apps. Now claims to be capable of searching certain other apps, but I don't see it happening much. If I click on a Now card for route information for the TTC stop I'm nearby, it opens details up nicely in Google Maps. But if I search for the actor who played Alfred in The Dark Knight Rises, it finds that information on the internet, rather than searching the IMDB app (even though IMDB is on its list of searchable apps). While Now was able to identify that I'm a Blue Jays fan from my Google searches, clicking the box score on a card for a Jays game loads the MLB mobile site, even though I have the official MLB At Bat app on my phone. I think that's a change that Google and app makers will make together going forward, it's the biggest flaw I see with Now at the moment.