I use open office and have found it very useful. I wrote my Ph.D thesis on it so even if you are planning on writing the great Australian novel, Open office will probably do. It even comes at the right price.
i used open office under linux and office 2007 in windows, the new open office supports the new office filetypes as well. in order of preferance i would have to say office 2007>openoffice>other ms office versions. also used corel word perfect, was pretty much the same as office xp at the time. open office is very powerful and a viable alternative to microsoft office imo