If you're talking about post-commentaries (recording the gameplay first, then voicing commentary later), I'd say those are interesting only if the person actually has something interesting to talk about (such as reviewing the game or pointing out strategies).
Live commentaries are good in that the viewer and the player are both experiencing the game at the same time, meaning that nobody is sure what will happen. For that to be interesting, the player has to have some entertaining improvisation and a few relevant topics to discuss.
Also, I find the group live commentaries (such as the Yogscast or Rooster Teeth) to be pretty entertaining. So keep that in mind if you have some funny friends.