For MS Office I only have the '03 Basic that came with my original Dell machine, so just Word and Excel. OO gives me an entire suite that I otherwise wouldn't have (especially Impress/PowerPoint). Personally I hate the new ribbon in '07, and have no desire to upgrade.
The 'Print to PDF' button in OO was originally a plus, but is now redundant as I've since installed CutePDF. If you're anal retentive about file size, OO's native formats are usually smaller than MS.
I do quite a bit of spreadsheet work, mainly to do with file format decoding and such. The extra graphing features I needed was only because of school, and since that's over with I don't really see myself reinstalling Excel whenever I reformat. The issues I had with .doc conversion I believe I can bypass without using Word itself, it was just faster that way.
I'm also a big fan of open source projects, so I tend to lean towards that area anyway. At some point I intend to release my X-wing series editors and library files as open source.
$do || ! $do ; try
try: command not found
Ye Olde Galactic Empire Mission Editor (X-wing, TIE, XvT/BoP, XWA)