After backing up the contents of my Kindle in two separate places:
Update 3.0.3 went smoothly; took about 10 minutes:
- after selecting 'update your Kindle' : 3 seconds to ' software updated'.
- restarted with green light
- rebooted
- showed: Kindle software updating, then ' software updated'
- restarted ' please wait a moment while your Kindle starts up' , with green light
- rebooted and started refilling collections
Effects on my Kindle so far:
- faster pageturning, more intense (?)black flash than before
- browser: much faster.
- bookmarks work better: don't have to log in to my gmail every time I open it, nor on my Mobile Read account. Dropbox synchronizes well, but doesn't open a book to read (as before) for ' opening multiple windows is not supported'.
- I am impressed by the speed of the searchfunction, which in my case was always the weak point. But now it really is fast, outside or inside a book, searching in google etc.
Sofar, all the updates speeded my Kindle up. I was satisfied with the 3.0.2. and hesitated for a second wether to update to 3.0.3., but for me it's worth the effort.

edit; don't use facebook/twitter on my browser, so can't report anything on that.