My experience with CM10 on a Nexus S 4G
I recently installed Cyanogenmod 10 on my Nexus S 4G and was able to use it extensively for about a week so here's my experience:
1. LAG. A lot of lag everywhere.
The worst one was in navigation. It starts when you're doing a voice command to navigate. You hit the microphone icon and wait about 5-6 seconds (that's a long delay) before you hear the "bell " sound indicating you can talk, but if you start talking then it won't register (I tried). So you have to wait further for the red circle to start "bouncing" around the microphone icon. Then you say, "Navigate to starbucks." (The voice recognition is actually not laggy) Then you wait...and wait...for the navigation app to start up. It finally does and after you finally get a lock and it finds GPS, it starts the route. That whole time is about 12-15 times slower than it was in ICS (CM9).
However, navigation gets worse. While navigating, there's considerable lag and the triangle frequently does not move (I never had an issue with ICS) before suddenly jumping to position. The voice sound also frequently plays only part of the sentence and skips a fair amount. This makes navigation a horrible experience and not very useful. You will miss turns.
Going to the homescreen created considerable lag as well. Every time I went to the homescreen to access the app drawer or a widget, the screen would go black (my wallpaper is black) and it would sit like that for a while before finally slowly drawing the widgets and icons on by one.
2. Battery life was significantly worse than CM9
With Deep Idle in CM9, I'd say the battery life was about 5% worse than before I switched to CM9 (I had stock ICS). However, in CM10 the battery life was about 15-20% worse than CM9 (even with Google Now off and NFC off). Over night with CM9 I'd lose about 25% battery life while it sat doing nothing (and there's a great signal here). With CM10, it's about 30% loss.
3. Lots of app and OS problems
The Google Voice widget was empty about 50% of the time and didn't update even when it showed something about 30% of the time. When the widget was empty, I had to open the app, hit refresh and then the widget would update with data.
Entering data into anything (i.e. using the keyboard) would frequently lag as key presses would take 3-5 seconds to react half the time.
A number of apps (pretty much everything at least once) would force quit on me.
All of those things are so bad, it makes CM10/JellyBean unusable for me. :(
All the above is too bad because JellyBean (and CM JellyBean) on a phone is the greatest Android OS I've ever used. It's significantly better than on a tablet. Google Now on my phone (before I had to turn it off) was incredible! It actually did help me out with routes and updates for my commute and plane information (as well as sports)! I usually hate privacy invasion but unfortunately, I have to admit it was great having a (limited) assistant.
Notifications are also much, much better on JellyBean. Email notifications are no longer useless and a mystery (on ICS you get: "You have 24 new emails." - from who? about what? do I want to waste my time or are they all useless?). On JB, you can see quickly who sent emails and get a quick overview of what it's about.
It's impossible to use CM10 on this phone as your daily driver. You'll be less efficient, the phone will get in your way and it will die by 4pm.
I'm "waiting in line" for the Nexus 4. ;)