How "heavy" do you use the suite elements? Word, Excel, Powerpoint? A free suite that works at about 90% compatibility for medium heavy formatted documents (legal) is Libre Office or Open Office aka NeoOffice. It is more compatible than Pages, Numbers and Keynote.
Not sure about the latest versions of Office, but there have historically been small incompatibilities between OSX and Windows on Word documents, something like 95% compatible, kind of like .docx and .doc on windows.
If you just do letters and memos, Pages is probably fine. Cant comment on Numbers vs. Excel because I'm not a power user.
Dont see too many people actually collaborating on .ppt files and Keynote is very user friendly, so it may be a perfect substitute.