IBM Notes 9 Public Beta is available (I downloaded it here), and there is a lot of good stuff in it.
Here are some things to watch out for from a user's perspective:

• Inbox can be categorized into "today", "yesterday", "Rest of Week", "Last Week", "Older" sections.
• New powerful find dialog (CTRL+F).
• You can scroll down the monthly calendar view to get to the next month.
• Opening a link in edit mode (CTRL+Click).
• Policy to automatically set the date sorting in the inbox to "most recent on top" or "most recent on bottom".
• New mail slider instead of the boring popup.
• Close original mail when replying/forwarding.
• Message snippets right in the inbox.
• Coloring of calendar entries depending on their category.
• The "check calendar" dialog is no longer modal, that means you can check the calendar while doing other things in Notes.
• Improvements in search, for example you can now search the mail file and the archive at the same time.

Beside these small but important changes, there are the big features like:

• Tighter integration with Connections (Activity Streams).
• Do stuff right from your inbox ("Embedded Experiences").
• Updated UI, and very similar UI between Notes and iNotes.
• Notes as a browser plugin.
• Support of the OpenSocial standard.
• Various improvements to the XPages engine and Designer.
• The XPages Extension Libary is now included by default.
• REST  Java, LotusScript and C APIs for calendaring and scheduling, meaning that third party tools can work with Domino calendars.
  (REST APIs are planned)
• Notes Traveler supports Windows Phone 7.5, Windows Phone 8 and Blackberry 10.

And finally my impression is that Notes 9 again has various performance improvements. It still is a fat client, but feels more responsive than any previous Eclipse based Notes.

Update 1: according to Karsten Lehmann, Domino 9 also delivers Dojo 1.8, which brings important improvements for Mobile Web Applications and much more.
Update 2: The C&S APIs are currently for Java, LotusScript and C. REST is planned.