You can determine that yourself by querying the bug tracker. http://bugs.contribs.org/ then choose 'Matrix'.
I am not sure how I can determine that just by looking at matrix. All I see is that the number of bug reports for SME Server Release 8.X is very small as compared to SME Server Release 7.X
But that can be one of two things, SME8 is very stable and does not have many bugs in it, or not many people are testing it.
But I can't tell which one is the case just by looking at matrix.
If latter is the case, I believe you might get more people participating if a forum was created for SME Server 8.X
I know developers really want people to post problems at Bugzilla, but most people prefer to check with other people: "Is this a bug or did I F... up somewhere" Before posting a bug. Because at a bugzilla, if it is not a bug, only answer you get there is "this is not a bug, closing"