Many sites report that the screen is now brighter since the update, but yet uses less power.
If you don't see multiwindow, you have to turn it on.
My dissappointment is the poor numbers of apps that can use this feature. The most practical apps for this would have been the Samsung Calendar, so you could bounce between email and calendar for booking. But, Samsung has yet to release their apps as multi-window friendly, despite the fact that many apps were updated for various enhancements.
This is a good video showing all the changes in 4.1.2: