I've been using Office 2007 on Vista for a number of months now, and if there is one product that I've re-fallen in love with, it's Groove. Simply put, Groove WILL change how you work - it has for me and I suspect that once the rest of my team starts using it, I'll even see more value from the product.
Groove isn't new - I've been using it for years before Microsoft purchased it. Groove is Ray Ozzies brainchild and was clearly ahead of its time when it was originally released - and clearly ready for prime time now.
(this was not a paid advertisement <grin> I really feel this way )