I really like the UI. A lot of people have criticized MS for breaking their own UI standards, but who cares? And if anybody should be able to break the standards, it should be the party that set the standards. I've given Outlook, Word, and Excel pretty good workouts, especially Word. I had a long design document I needed to finish and a shorter one I needed to write yesterday, and did them both with Word. It is true that because the UI was drastically changed, you will find yourself looking for things that you used to know where they were. However, the UI makes it a lot easier to find those things. And that's really a power user problem. The most common stuff is even easier to get to than ever before. Power users will adjust to the new menu layout. If you were able to find the "Update TOC" for the first time on Word 200x, then you will certainly be able to find it on Word 2007.
For Outlook, I like the To-Do bar, especially showing the next several events on the calendar. It's UI is otherwise less changed than Word or Excel. Excel's UI is similar to Word. It make nice use of contextual menus, so when you select a row on the sheet you get a menu with formatting options, as well as the normal contextual menu you get wheny ou right-click. This is true on Word as well, but seemed even more useful on Excel. Or maybe it was that I had gotten used to on Word already before I used Excel. Either way, this is good feature. I also liked the new tab button, similar to IE7, at the bottom of the window.
I haven't tried Power Point. I installed Visio, which I use a lot. It has a lot less changes, and seemed more like a modest improvement over the earlier version. Like all versions of Visio, the file format is different. This is true with all the Office apps, but is particularly annoying to me when it comes to Visio. I also tried OneNote, a program I use a lot at times. It's UI is very nice and its integration with Outlook is great. I am going to try its online mode for sharing notebooks. I've been using Google's Notebook for clipping ads for houses (I'm moving at the end of June) and sharing them with my wife, and it's very useful.
Now for the negatives! These programs are very slow. Very slow. I don't know if that's because they are beta, or if they are just going to require more CPU power in general. I've been running them on my laptop, which is 1.8 GHz Pentium M with 1 GB of RAM. That would seem like a lot of CPU for Office, so I'm hoping it's just because they are beta. They also require Microsoft Desktop Search to enable some of the features. That's an interesting turn of events. I've played with Microsoft's search in the past and liked it, but always used Google's instead. I could certainly see this Office 2007 feature causing MS Desktop Search to gain traction. Of course it's going to be included with Vista anyways.