The incompatibilities between mac office 2008 and windows office 2007 are enough that I won't use Mac office at work unless it's only to read files. I won't ever edit a file using Mac office that others will have to read. Part of the reason is because I work with so many people who insist on using excel macros that use VBA. Other reasons are because I've been bitten by formatting issues. I also don't use Mac office for any very important documents that others may read (e.g. a resume where the employer insists on a .doc file). In fact, for unimportant personal docs, I now use google docs. For everything else I use Windows Office. If I had to buy one or the other, even as a mac user, I would buy Windows Office and run it in a VM or through bootcamp depending on how often I have to use it.
Yes, Mac office is usually good enough, just like neo-office is usually good enough, but I've been bitten too many times that I don't take the chance anymore.
I really wish Mac office was truly 100% compatible with its Windows counterpart. Unfortunately that will probably never happen.
As for how Office 2007 will run in a VM, well, it will run fine but not as fast as a straight up windows machine. If I had to spend all day at work using office, I would greatly prefer to run Windows in bootcamp (or on a regular windows laptop) rather than use office through a VM. Of course, this is just my opinion and others may say that using it in a VM is more than fine.