All the people I know with cross-platform experience w/Office say Office:Mac is a really good product.
I don’t know myself. I’d type out formatted documents in raw RTF or HTML in a text editor before I’d use Word, I absolutely detest Word. And have never found a purpose in life for PowerPoint (as with GarageBand, it’s just one of those apps I have zero need for).
I can only speak for Excel. On the Windows side, you get OLE support. On the Mac side, you get AppleScript. Advantage: MacOS. On the Windows side you get deeper levels of keystroke equivalents for useful commands, while the Mac version is cleaner and feels more streamlined but more often requires you to use the mouse. Advantage: Windows. It’s a great piece of software on either platform, probably Microsoft’s best product overall. For the most part, you can edit the same workbooks back and forth in Windows and MacOS, and for the basics you’ll easily forget which platform you’re using because it won’t make a difference very often one way or the other.
If you’re an Access user, be aware that there’s no Mac version at all. (I’ve heard that there’s a Mac version of the Access-equivalent in OpenOffice, and that’s it’s compatible with MS Access and can use its files, but I’ve never seen or used it). Mac folk use FileMaker, which is a better database on both platforms anyhow.