I've been using it for at least 5 years to watch Sox games. MLB has a policy that games on mlb.com can't interfere with local teams coverage of games. I lived in Fairfax County, so that meant Nationals and, far more often, Orioles games would be blacked out. This was fine because I could get them on the local TV station.
Now I live in Greensboro, NC and have learned that despite the much greater distance from Washington and Baltimore, I am still blacked out from watching their games even though there is no other means (TV or otherwise) of watching their games down here. Plus Atlanta--of course-- is on the blackout list.
What makes all this worse in my opinion is that there is also no possibility this season that I could attend any of these games in person. In other words, MLB and especially the Orioles and Nationals franchises are throwing away a viewing customer to whom they could show commercials (on mlb.com) between innings and when pitchers are changed during an inning.
One more thing. mlb.com offers two feeds for most games (that aren't shown on cable by ESPN or Fox): the home team's and the visiting team's. Given that, I wonder why they don't just blackout the Sox feed (NESN) when the Sox are playing the Braves, Nationals, or Orioles.
To me it's dumb, but what do I know?