After seeing this demo, thought I'd try out contenteditable for table manipulation.
Pros: Effortless editing of cells, keyboard navigation with arrow keys, buttons to add and remove rows and columns.
Cons: Adding rows and columns breaks th usage. Copying and pasting multiple cells creates a sub-table in the cell where you paste it.
Really impressive - it offers a lot of functionality just by setting one property, but I'm looking for more robust manipulation, mimicking that of Excel, so I'm not sure it'll work for me. Plus I need to support IE7. =P