I found a little dated article comparing OpenOffice.org 1.0.3 to Microsoft Word (through Tom Vergote) which is quite interesting:"OOo Writer outperforms MS Word in almost every way--by a wide margin in its implementation of styles, and by a narrower one in general stability. MS Word's main advantage is the ability to reduce drudgery by recording macros.
That feature alone is probably enough to make some users stay with MS Word for the next few months. Depending on your work habits, you may have other preferences. For that reason, I suggest that, before evaluating OOo Writer, you make a list of the features you like or dislike in MS Work, then go looking for them in OOo Writer before you do any serious work." (The macro recording feature mentioned has been included in OpenOffice 1.1 by the way)
For a report I had to do a couple of days ago I first installed and used OpenOffice 1.1 (the PDF export feature really rocks) and I actually quite liked it. I heard rumours that OpenOffice saves it's file as XML files, however the .sxw files it produces don't look like XML at all. Then I read that .sxw files are actually zip files containing xml files:
META-INF/ contains information about the document Pictures/ contains images I used (in it's original .jpg format) content.xml contains the text itself in a neat XML format meta.xml contains more meta data settings.xml contains editor settings styles.xml contains styles
That's so cool, you could quite easily translate these files to another (XML) format. I know the DocBook export feature in OpenOffice is done this way, just with a XSLT transformation.
The article also contains a PDF containg a feature-to-feature comparison.