I'm coming from ML2. Flashed via ODIN without wipe.
Everything moves a lot faster! Animations are smoother and the phone is a lot more responsive! The Gallery opens up a lot faster! I think a lot of the minor bugs have also been fixed. I haven't had time to check the audio bug (where audio stuttered during playback). The music player is more responsive, too. Plus, my battery life has been drastically improved. It's also not forgetting the last opened application
On the downside, re-opening background applications still isn't fast. It takes a while due to Touchwiz hogging all the RAM. I guess I might be used to my Nexus 7 because that opens stuff instantaneously. Also, there is some bloatware on the phone but the applications can all be uninstalled without any problems!
Overall, it looks like a fantastic update. Highly recommended!