I've recently spent some time with Office 2010. Because I have an MSDN account, I usually get access to a lot of stuff from Microsoft prior to public release. I think the actual release date is tomorrow, June 15th.
Today, June 14th, I uninstalled it.
Several of my issues were rather minor. I felt that the file ribbon was a bit freakish in all the '10 apps. It is a bright and overly vivid green. This is a petty complaint, but distracting nonetheless when you are trying to get work done.
Outlook 2010 also incorporated the ribbon approach to the user interface. That was no problem. But the UI has become a box, with a colored "borders" around the entire application. This leaves white space in the middle and the email content there. It's like you are looking through a whole in the wall. Again, it's not a very welcome UI for something you tend to work with all day.
The real kicker though became the fact that the combination of Outlook and Excel suddenly decided that they couldn't open Excel files from the 97 to 2000 era. That certainly became an issue for me - within hours. I don't recall when it started happening or if it always was, but that just made it a no go.
So I uninstalled it and decided to go back to 2007. That was also apparently a mistake. Outlook 2010 did not uninstall or remove the Mail setup it had created on installation. When I went to install Outlook 2007, it could not handle the fact that Outlook 2010 (jealous anyone?) had been there and apparently Windows still only supports one Exchange account.
The solution to that at least was a registry change. You'd think in this day and age, what with Windows 7 and so forth that we wouldn't still be stuck using regedit. Maybe that'll be in the next version.
In the meantime, if you are running 2007 I recommend you stick. You'll do just fine. I suspect folks looking at Sharepoint 2010 would find some benefit, but others not so much. I'll wait for the next version at this point.