I’m not nearly as familiar as Guy is with Groove, but that never stopped me from being contrarian before, so…
Before you jump into the Groove-y crowd, you might want to read Robin Good’s article “Groove: Ten Reasons Not To Buy.” Far from being a juvenile rant about how the program sucks, it’s actually a well-thought criticism of the usability, features and interface of the program from someone who’s used it extensively. Good also goes on to suggest several improvements, including this one:

“Having the pervasive ubiquitousness of Microsoft to support its development I really wonder what prevents Groove from disappearing behind a button inside Word, Outlook, Powerpoint and Excel. Wouldn’t that be easier than firing up again another mammoth application?”

The answer to that question is this: because then it might be too similar to Sharepoint Team Services.