Most things export/import OK. Fancy formatting can get lost (for some reason if I paste a graph from “Open Office Calc” in “Open Office Writer” and save as Word.doc some people can’t see it (but not all) and I also get problems with pasting graphics (e.g. a flow chart thrown together in Open Office Draw and pasted in). Likewise things like vertical text in excel spreadsheets and “forms” give headaches in both OO and Google docs.
My personal preference is Abiword, and Gnumeric – but not sure if they are available on Windows easily. They offer almost as good cross compatibility and are, for 99% of the things people do, much cleaner / nicer to use.
The killer is is you need anyone else’s ‘plugin’ modules, macros etc as most won’t work.