I updated my Nokia 603 just now, below are my personal experience.
1. New Swipe to unlock feature is really very nice.
2. New Music player
3. Menu effects are very good
4. Scrolling in gallery is very fast now
5. We can double zoom the image now
6. New Nokia Music app really very fast.
7. New widget (New mail arrivals is very good looking)
1. Left right key in key board is no more available
2. Image arrangement in gallery is reveres now (new image comes in end)
3. Battery drains very fast
belle was outsourced, so thw whole development is handled by accenture.