I've been using Office 2007. Ugh. I have a headache. Heartburn too.
Maybe the new menu structure/ribbon thing will be worthwhile someday, but it'll take years for me to recoup the productivity loss from this transition.
It varies by application. Word was such a miserable product that the ribbon is a minimal hit. They appear to have made some stab at fixing Word's completely broken stye sheets, but the "themes" tool requires use of .docx (surprise!) and that's not practical. Maybe in 3-4 years.
Access appears to have been severely wounded by the ribbon. Does Microsoft really think there's a way to make Access pretty? It's a data hacking tool for heck's sake! The new Access has some nice Sharepoint integration, but I don't see anything else I like so far. They don't appear to have fixed the big problems with Access (inline functions don't return values, rather the cell contains a pointer to the function, and the links to tables are still absolute paths and break all the time). Excel, as usual, escaped the worst of it. Nobody dares touch Excel.
Then there's Outlook. Ye gads. It may be the worst of the lot. Damn ribbons - I need a 32" display now! There's only one good thing in the Outlook update (excepting Sharepoint integration) -- the category view no longer breaks whenever you sort on a field. That bug has been in Outlook for eons.
Only a monopoly could get away with something like this ...
Update 6/26/07: Some things look bad, but get better over time. The ribbon isn't doing this. Ctrl-F1 toggles ribbon display of course, but that only mitigates the mess. There's something called a "Quick Access Toolbar", it feels like it was added in a last minute panic. You can partly restore some basic usability by painfully configuring the QAT, including tediously ordering the list by clicking, incessantly, an up and down arrow. That's right, no drag and drop, clicking an arrow. Vintage 1989. If the OpenOffice team tries to emulate any of this stuff they're insane.