You have to pay for it, but I have nothing but good things to say about mlb.com package for online viewing of games. If you want to split the cost with a friend I'm told they don't check to make sure only one person is logged in at a time, though I haven't tested that extensively.
Seems my post didn't get posted...
I am not a fan of mlb.tv due to blackout restrictions. If your team plays on Saturday afternoon at 1pm, and the local team in your area plays at 4pm, your game will be broadcast in oblivion due to exclusive Fox rights on Saturday afternoon.
Since I only want to watch my team, I have no interest in mlb.com video apart from watching my team. The inability to watch them when Fox is broadcasting Red Sox games on Saturday here in the MA market led me to give up on it.