I have been using Office 2007 since it came out in Beta 2 and one of the coolest things so have seen so far is the "Find Duplicates" button.
When is comes to data cleansing this will be a God send, combined with the ability to have up to (i think) a million rows. Just think of the number of times you need to populate, say a list of cost centres, for a dimension, only to find that what you think should be a primary key is not! Now you can just throw it into Excel click a couple of buttons and find the pain! Ok its not a long term cure for an underlying problem but what an awsome tool to use to find the cause and then eradicate it.
The second best is the Data Validation button. Again the ability to throw data with something doggy in it into excel and start hunting the bad line will be a HUGE saving.