I had a little bit of trouble with little silly stuff in the beginning, such as where the undo button was! But I am super happy with it. I especially like that you can customize the toolbar in the upper left corner with buttons that you use the most.
Some features took me a while to find, like the form locking... but now that I'm used to it... it is definitely much more functional and easier to use... We use 2003 and 2007 and we desk-hop to make sure we don't get too used to one thing... but even I found it frustrating for a start.
Filtered merges I have found are too smart for their own business. It appears to recognise excel fields a lot more so I am back to the drawing board on one merge I completed... either that or I did it wrong :)