(UPDATED 9/21) The official Verizon update has arrived! Users are reporting the update is now available via Wi-Fi ONLY for the Verizon Galaxy Nexus. Get on Wi-Fi and hit that 'Check For Update' button now!
I installed the leak on my rooted GN on September 5th and have been running it since. I thought I'd share my impressions here for those who may have grown impatient.
Jelly Bean definitely shows marked improvement in the responsiveness of the phone in terms of swiping, scrolling, and opening apps. Google Now offers some really intriguing features, but at this point for me they offered little value. The updates on sports scores were nice (at least once Google got NFL scores going for week 2!). I imagine people who do a lot of travelling would love the flight updates and travel features, but as I rarely move about the country (and even less about the world), they were of little use to me. Same goes for traffic updates - I work from home so traffic notifications offer me no help. I tried to activate "places" to get updates on nearby events or businesses but I never had a card show up. Its possible I just hadn't given Now a chance to learn about me before disabling it. The real issue for me, though, was the seemingly large increase in battery usage attributed to map/location-related programs. Under Better Battery Stats, there were no fewer than 5 different processes responsible for 'waking up' the processor on my phone, some with significant usage. Location services have always seemed to use an excessive amount of background processor time which always bugged me, but the increase I saw with Google Now active was just too much.
(Update: I'm giving Google Now another shot but with limited location features to see if it works any better)
The most obvious other change I saw in Jelly Bean was the device settings page. Instead of the previous "accounts & sync" line, Android 4.1 brings all your accounts to the main list for easier access and modification. I really don't find this to be a valuable addition as I rarely ever find myself adjusting things like app syncing. I usually keep all but the minimum necessities OFF. I feel that this change didn't add anything and just cluttered up the main settings screen and forced me to scroll more to get down to things like 'developer options' and 'about phone'.
The most useful change in Jelly Bean is the notification bar. Aside from the font style and color change, the notifications are far more useful. Instead of a very limited listing of your notifications, Jelly Bean provides you with a very detailed list of what's waiting for you including the content of SMS messages and an extended amount of info from received gmails, not to mention the ability to call contacts directly from missed call notifications. It really makes the notification bar so much more useful.
As far as functionality goes, the radios did not appear to offer me any improvements over the prior versions, but then again, I never had major problems despite being in a borderline 3G/4G area.
[Note: If you were rooted on ICS and flashed the Jellybean update, you probably lost root and don't even know it. To regain root, copy this updated SU zip file to your sdcard and then install the zip through cockwork recovery.]