A lot of improvements have been made to Touchwiz but as you didn't use it before you won't necessarily value them. However, Touchwiz is now better than ever. You can add or swap apps on the base bar much more easily, widget management is much more slick (resizing compatible widgets). Moving from one home screen to the next is now in 3D.
The notification panel now contains even more information and there are more device function control buttons at the top, which now scrolls sideways. I now no longer need a widget for mobile data toggling, for example.
Bundled apps have been improved, like the mail client which has added functionality.
As for 'project butter' I think this has improved things but there are caveats. Scrolling can be a bit sticky when the device is very busy, especially immediately after re-booting. But once it has settled down scrolling is definitely more positive smoother and more damped.
I haven't yet noticed any improvement in battery life although there are now more easily accessible ways of suspending apps and other functions which I think could probably make a useful difference to battery life.