I've now been testing the direct access to the MPP file with round trip data. Here is my current approach and results:
1) open mpp file
2) save as XML
3) read the XML
4) update and write XML
5) Merge back to mpp (not working too well?)
The results are in the file: RndTrpSaveAsXML.zip
Here are the new steps:
1) read the mpp file
2) update and write XML
3) Merge back to mpp
The results are in the file: RndTrpDirectMPP.zip
You will notice things like... resource calendars missing names... some extra things being added.
These tests have been performed using MS Project 2002. I'm getting reasonable results with the XML version. If you find that file files work ok with 2003 or 2007, please let me know.
(resources are reading ok).