MacTech benchmarks the new Mac Office 2008 and provides the following conclusion:
In general, Office 2008 is faster than Office 2004 when run on Intel Macs. And, on the PowerPC (represented by our PowerBook G4), it certainly runs "well enough" albeit marginally slower.
Microsoft hosted a day-long conference on the new Office 2008. Attendees will reportedly receive a free copy of Office.