If you’ve been happily using Office X for OS X and are thinking about upgrading, I have one suggestion. Don’t. I did and I’ve had problems with every program.
Word 11: For some reason even though the new document compatibility panel was introduced, Word 11 is less compatible with some documents than its predecessor. This is the same document opened in Office 11 vs Office 10. It’s a simple RTF document that even the built in TextEdit handled fine. The image pretty small, but you can tell that in Office 11 numbers are scattered all over the page while in Office 10 everything is laid out nicely in tables.
This isn’t the only document this has happened with either.
Entourage 11: Entourage 11 has some nice new features, the project center and a nicer layout for reading mail. Entourage also crashes all. the. time. At least for me. I know what the problem is, for some reason I keep getting a large message and Entourage can’t handle it, so it crashes. I have to open Mail.app, get the message, delete it and re-open Entourage. In this case I can’t vouch for Entourage X. I haven’t tested it to see if it works fine there.
One way Entourage 10 is better than Entourage 11 is in it’s Exchange server capabilities, at least with older versions of Exchange. In Entourage 10 I can connect to the Exchange server and get mail (I can’t send for some reason) but with the newer Entourage 11 I can’t even connect to receive mail. The support got worse instead of better. To send and receive I have to go all the way back to Outlook 2001 for Mac.
Powerpoint 11: At work we use Powerpoint for storyboarding. It’s important that graphics and images stay exactly where they are placed. Up until I upgraded to Office 11 I never had a problem getting storyboards from designers working in Office 2000 for Windows, since the upgrade I often find that images and shapes have shifted from their original position. Usually the shift is a move between 5 and 20 pixels on the Y axis. It’s enough of a problem that I’ve switched back to Powerpoint 10.
Excel 11: I haven’t found much wrong with Excel, other than a few new “features” that I have to find a way to turn off. For example, (this is across all Office 11 apps) whose idea was it to make the formatting palette become translucent after a few seconds? What does this improve? I find it plain annoying, and even more annoying how it snaps back to 100% rather than fading nicely in. I disabled that “feature.” 
I also have found that Excel 11 has a bad habit of pointing out every possible problem, even when they’re not problems. If you have a list that is summed up at the end and there are blank cells (like for rows you plan to fill in at some point in the future) you get a little warning sign that just sits there until all empty cells are full. It’s not a big deal, but as far as I’m concerned, I’ll just stick with Excel 10 and avoid having the program correct my non-mistakes.
From my experience, I’d say save your pennies and stick with Office X for Mac. The upgrade has caused nothing but problems for me.