Besides Office, my 1990 backups also contained several Microsoft Works files. Here are my notes on opening those files in a modern Mac environment:
Opened on the first try in Excel 2004 for the Mac. NeoOffice barfed on them.
Word Processor (*.wps):
Opened using "Recover text from any file" in Word 2004 for the Mac. Accented character encodings are shot, obviously. NeoOffice barfed on them too.
Database files (*.wdb):
No success. A flexible text editor like vim or TextMate will let you see the file contents, but there is no real way to dump the tables.