The C't magazine did an extensive test on Office applications about a year ago, and the only seriously negative points they could find about OpenOffice were, that documents saved as html pages sometimes didn't display correctly in IE, and that sometimes importing a MS Office document changed the layout. Although there were a few minor ones as well, that were mostly about it being hard to find where to change things, like spell checkers or the default page size. Almost all of those (exept the default page size, AFAIK, which still took me Googling) are fixed in OpenOffice 2.
Microsoft Office, on the other hand, had quite a lot of serious issues. For example, when using headers and footers and at least one picture or other embedded object on each page, it was impossible to create a 100 page document. Word crashed every time. And things like multipage tables, different paper sizes and orientations and changing the header and footer often all tended to crash Word and destroy the document after some time. Very large spreadsheets also suffer from that and sometimes lost links. And (IIRC) a html page saved in Word was much bigger and only displayed correctly in IE.
In general, when you want to work with large and/or complex documents, you better don't try that in Microsoft Office.
The best and worst thing about OpenOffice 2 is, that they copied the UI from Microsoft Office for more than 90%. That makes it much easier to switch, but unfortunately that makes a lot of things, like wrestling with the layout of spreadsheets with embedded objects just as much of a pain as with Microsoft Office.
Still, it didn't crash even once during the test, and it does it all as promised.
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