“familiar spreadsheet-like interface, coupled with file sharing, work automation and Gantt chart features have made it a popular and highly functional online collaboration tool for teams”

See also: http://www.smartsheet.com/announcement/Smartsheet-Raises-26-Million-in-Funding-from-Insight-Venture-Partners-and-Madrona-Venture-Group

What makes this spreadsheet-in-a-cloud better than Google Docs?

"Today, more than 1 million people across 100 countries use Smartsheet to coordinate and manage processes and projects by sharing critical information securely with colleagues, vendors, and clients. Its easy-to-use spreadsheet-inspired design and cloud-based architecture is revolutionizing how groups of all sizes can work effectively together."

Unclear.

Well, ist's a spreadsheet-like UI for a Project planner, not a spreadsheet per se. I'll admit that some of their use cases are about versioning hell with Excel and email, though…