Yes, it's a spreadsheet, but what a spreadsheet! The designers at Apple started from the iWork basics -- a blank canvas onto which you can place content that you control. One form of content is a table with all the spreadsheet tools you expect such as formulas and computations. But that canvas can also hold text images, and active hyperlinks. By starting from the solid iWork framework, all of these tools work together.
Of course, the design of the built-in charts and other graphics are consistent with the other iWork and Apple graphics, which is to say, they're beautiful. Just browsing through the templates that come with Numbers you'll see examples of things you never thought of doing in a spreadsheet.
Just as Keynote reinvented presentation software, Numbers reinvents the spreadsheet. Find out more in iWork '09 for Dummies.