Fr. Sobrino is one of the leading minds of Liberation Theology. LT, in my opinion, is a reactionary ideology to the church becoming cold and calloused to Jesus' calls for true discipleship. LT has a great deal of really good ideas that cannot be dismissed simply because of the Marxist overtones. I believe with the South American political climate becoming increasingly leftist, we'll see a resurgence of Liberation Theology.
If you want some good primary source material for Liberation Theology, I suggest reading:
Gustavo Gutierrez, A Theology of Liberation: Perspectives
Jon Sobrino, Christ the Liberator and Jesus the Liberator
I'll post more about this in the near future. Also, one of my professors did his dissertation on Liberation Theology's Christology (which I read last fall for a bit of an intro to the subject), so I may see if I can get him to write up a guest post about Sobrino, his ideology, and perhaps where he sees Sobrino and LT going in the next few years.