Just started using Mathematica 8. My initial impression is that this is a fantastic upgrade. The free-form input is amazingly useful.
The notebook interface preserves the familiar look, but the font rendering makes it appear cleaner. The input-selection feature is very useful, as is the seamless integration with Wolfram/Alpha.
I also like the splash screen, which in addition to the announcements displays a list of recent files that one can use to open any of those files. This eliminates the need to use the File menu to open a file. The splash screen is actually useful! Imagine that.