Just to be clear I like R12, it's got a lot of the benefits folks mentioned above but there are some drawbacks in the current version (we just upgraded from 11 to 12 a month ago). Stephen captured a few of the gaps from 11 in his note so i won't duplicate here.
-We saw a major overall improvement in calc time (Yay)
-Significantly improved cross-application navigation
However...
-Waterfall Charts require TBM Studio access to add notes and Check-in's which will kickoff calc's so it's not really conducive to collaboration with Service/budget owners anymore (related to the Editable tables section Stephen mentioned)
-Performance issues in the TBM Studio screen have led to a lot of support calls for us since upgrading, need to refresh the screen fairly frequently and we have seen a lot of sync issues.
-Export to Excel is currently broken for non-TBMA's on a component level (whole dashboards do export)
-OOTB Component reports in R12 are not updated from the later R11 content, the Dashboards aren't exactly role/audience aligned or domain/use case aligned it's still a blend of everything without an overriding theme or taxonomy.