I got have both, and I tried to use iWork for school but have found it to be insufficient.
My professors use Windows/Office, and whenever they upload a file, and I try to open it with Pages, there is a formatting error. It will still open the document for the most part, and you can still read it, but sometimes the formatting is off and it makes everything messy. On top of that, it will always prompt you if you want to review the formatting errors everytime you open up a document.
I got sick of this, and just switched to Office 2008 for mac. I haven't had any problems since.
So if you depend on using files that other people create with Office for windows, I would still recommend getting office for mac.