I appreciate the first two comments. 'Numbers' will not be feature-laden enough to compete with 'Excel.' Those who are choosing which spreadsheet to buy will—because MS Office is "entrenched" and "works reasonably well"—be inclined to prefer Excel.
However, the essence of a 'halo app' may not be that it can compete directly with an established, well entrenched software application.
(1) 'Numbers' may be innovative, so that it approaches spreadsheeting in a new [efficient/fun/focused/purposeful] way, and (2) 'Numbers' may fill a gap (i.e., spreadsheet) that will allow potential Macintosh customers to be comfortable enough [with iWork] that they will go ahead and make that Mac purchase instead of running back to Dell.
In this sense, 'Numbers' could indeed be a 'halo app' if not a 'killer app.'
Is the Killer App Dead or Will Apple's Numbers Revive It?