I just switched companies, but at my previous company I switched to 9.0 about 2-3 weeks after the release, my new company is still on 8.3 but it seems the plan is for me to do an upgrade within the next month or so.
After the 9.0 switch, I did not have any issues, but we didn't upgrade the workbooks unless edits are being made. So the server was upgraded to 9.0 and all new workbooks are created in 9.0, any of our previous workbooks we kept in 8.x unless edits were made and then the workbooks were converted to 9.0. I didn't hear of any workbook issues during my time while I was administrating 9.0. I'm sure some folks have very specialized issues, but after going from 7.0, 8.0, and 9.0 I haven't seen any major issues that would cause a complete rollback.
A few things to note for anyone that reads this:
The server interface for 9.0 is fairly different
Make sure to do a backup before upgrading
Tableau likes to change the path C:\xxxx\Tableau\8.3 C:\xxxx\Tableau\9.0. I find making a generic directory sometimes makes scripting (tabcmd) easier to manage
With Tableau Server you get a developer server license, so if you have a lot of client reports you can test before making an upgrade to a production server
Let me know if you have any questions, I'll be happy to help. When I get a chance I'll try and share some of the Tableau Server work/scripts that I have done.