Excel and Access are the tools that are toughest to replace. IWorks is getting better and does a nice job, but heavy data crunchers still need Excel and Access. Apart from this type of user, the iWorks suite is easy to use and ever more compatible with office applications.
Personally, any word processor works for me and my company. We need Excel to continue to use legacy macros and vba automation in our more complicated spreadsheets, but for everyday spreadsheets or for executive reports or Board reporting, the spreadsheets are typically more straightforward with simple cross tabs of data and a chart or two along with some narrative. Numbers can generate these and output to Excel or .pdf. Keynote is an excellent PowerPoint replacement. Again, nothing can replace Access and that's probably the reason MS won't make a Mac version.