I've used Word 2001 extensively. Only problem I've had is an occasional dialog that pops up presumably when Word is attempting an auto-save that says something like, "failed on save"; if I "retry" the save (the default button to press), everything seems to go fine. Oh, and once in awhile, the cursor disappears; it either disappears until I return to a native OS X app (like the Finder), or it disappears entirely (even in OS X apps) until I quit and restart word. I've used Excel 2001 a bit; it seems to work great. I've used PowerPoint 2001 just a little bit, but it didn't seem to have any problems. I've never even launched Entourage.
I have had no trouble using Office 2001 within Mac OS X. Do make sure you have updated Office 2001 with the service pack that is out. Look on versiontracker.com for it. Other than that, it actually works great running through the classic environment.