Jelly Bean is smoooth. Buttery even.
It remains to be seen how useful Google Now is. A big issue in Australia is that the weather forecasts from most Android-app-using sources (WeatherNews, etc) is a bit crap - there's some apps that use the Australian Bureau of Meteorology as their source and they're significantly more accurate. Unfortunately Now doesn't use that source, it uses the generic sources.
There appears to be tighter integration with other services - there's a section in Settings that has Accounts. The "add an account" offers Skype and Dropbox as first-class citizens. I assume that's only because I have the apps installed? I'd actually already configured Dropbox through 1Password, and Dropbox is listed in my Accounts list right next to my Google account. It's an interesting level of integration...
The incremental improvements to the notifications shade are nice. Less blue is good.
Oh, the Google Now background changes based on the time of day / weather. Nice
The voice search is really well done. Its recognition is far beyond the previous Android attempts, and the search integration is pretty good.
Interestingly, a few weeks ago my wife was in a store looking at some laptop covers so she asked Siri "what are the dimensions of a 13 inch MacBook" and it basically didn't understand the question. She asked several variations, all with no result. Just asked my Google Nexus the same thing and the response was the Google Search including the first result being a tech spec page for the MacBook with the dimensions.
I'm still not sold on voice search (I feel like a right twat using it) but maybe it'll grow on me...