I think as far as support for the legacy binary MS Office formats go, OpenOffice.org is your best bet. Support for creating those files is usually much better than for reading, so most compatibility issues you will encounter are usually on the "read a file created with MS Office" side, instead of "read a file in MS Office which was created by OpenOffice". Usually implementations aren't dead wrong but rather incomplete, so opening files might cause them to lose some features or fidelity, but creating usually is painless.
Other office suites you may want to try (although they don't have the extensive history and struggle for MS Office support as OOo) are KOffice and GNOME Office.
Also note that the binary file format specifications are all made public under Microsoft's Open Specification Promise so they're not entirely1 closed for quite a while anymore. Support will likely get better.
1 There are still some parts that require either reverse-engineering of previous Office versions or some guesswork2, but for the most part the specifications are complete and publicly accessible.
2 I also don't think most people need the compatibility "features", such as "line spacing as in Word 5.5" or similar so that's essentially a moot point for most practical implementations.