Key benefits (over comparable Rich Client development tools): high-security data replication/synchronization, and user identity management.
Cost: $50/seat (plus the cost of any 3rd-party application you might be running, which is the whole point).
MsWindows only, uses COM a lot.
Microsoft owns a chunk of them now.
I'm not sure what you mean by "serious" work but in the first preview we support C# and VB.NET, as a Groove tool revolves around using (now) a Win Form and those are the two languages with Visual Designer support. I could have added MC++ but that isn't supported by the Visual Designer in V1.
Massive integration with MsOffice, MsProject, and potentially SharePoint seem to be coming.. Complete MsOutlook integration is available for (that magic price point) $50 from http://www.parallelspace.net