I wasn't too impressed with my experience with word2x. It tended to munge even simple MSWord docos (you may get better mileage from new versions).

At work we are writing code to do exactly what you want, and we finally settled on StarOffice as the converter. We just dump the uploaded file to disk, call StarOffice to convert it and then use the output. It's not an ideal solution at >80Mb, but it's free and does the job.

HTH,
Jeremy