Chris Webb has written a good blog post entitled "Introduction to MDX for PowerPivot Users".  Worth a read if you use Excel PowerPivot and are looking to go beyond the basics.  The MDX query language is a powerful tool when trying to analyze data dimensionally.  With the introduction of Excel PowerPivot and DAX, a lot of people are wondering what the future of MDX is.  Nice to see posts like this from Chris as they show how the MDX query language is still very useful for Excel users.