Ever since Apple released Keynote rumors were circulating all over the fan sites as to when a full truly Mac office suit would be released. This was addressed bit-by-bit. First, Keynote was joined by Pages. Then both Keynote and Pages have received a small table management face-lift. Finally, this year Numbers has been released.
First great departure for many long-time Lotus 1-2-3 and MS Excel users is disappearance of work sheets. Sheets are still there, but they really refer to sections of your work book -- sort of like virtual pages in the file that may or may not fit a single A4 piece of paper.
At the same time you're getting the immense flexibility as to where and how you want to lay out your data. You would not notice this if all you need is a plain data listing (but then you may not be using the right tool), but if you, say, need to have details table and several summaries and maybe a chart or two to go along -- Numbers will really start shining for you.
Instead of providing you with a preset grid, you get just a page into which you can place as many individual tables as you like. This is the case of OLE finally done right, if you wish. And yet you can cross-link data in all these tables (since they act similar to worksheets of a traditional workbook).
This means that no longer do you need to play with column widths, merge cells or stretch your fields over expanses to achieve something like desired layout. And you get lots of samples of more or less typical spreadsheets to start using/learning/building.
Numbers also understands and interoperates with quite a few external sources/data formats. Excel is generally not a problem to import and integration with Address Book application lands a sweet spot for those who need to create simple lists/phone directories/etc.
That said, at this point it is very weak in the formulas support department covering only about 150 most frequently used ones. It lacks support for such "popular" data analysis tools like Excel Pivot Tables and filtering data is a bit of a bitch, since dynamic filters a-la Excel are also not available. Last but not least -- it is not blazing fast, especially once you start copying your formulas and/or data across the columns and rows.
Another bit of a problem and seemingly an oversight is the fact that you can't build a formula with keyboard only -- you *must* use a mouse or else key in cells and range names by hand.
It would seem that Apple could do much better following in the steps of KHTML to WebKit and back experience that allowed them to build a very good and standards-compliant browser quickly. I am pretty sure that both KSpread and Gnumeric could provide a great spreadsheet engine that already covers much more mathematical ground (especially Gnumeric).
Last note is on file format. When looked at on a console it is apparent that it is neither OOXML or ODF, but rather an XML dump of NSMutableArray Objective-C data. While this may have been done to speed the development and implementation, this will likely be a holding factor for going any further -- Apple would have to tackle on the interoperability issue with other spreadsheet vendors -- be it OOo, MSFT, KOffice or Gnumeric.