This data set was collected from GameSpot and consists of reviews of games from 4/2005 to 1/2007. Information on the rating system is available on the GameSpot site although it has changed a bit.
The GameSpot data can be found here.
The reviews consisted of a score, from 0.0 to 10.0 in increments of .1, an author and the text of the review. They were further discretized into 3 bins for sentiment classification as described below.
Further details can be found here.