Q. 1. My understanding is that the ordinations of the Society of St. Pius X are valid and that the Pope lifted the excommunication of its bishops. Accordingly, can I attend the celebration of the Holy Mass at the Society of St. Pius X in fulfilment of my Sunday Obligation?
A. 1. As you stated, the ordinations of the SSPX priests are valid. Therefore, the Masses are also valid. Technically, by attending the Mass of the SSPX priests, you would fulfill your Sunday obligation.
But is that the right thing to do?
I would recommend that you attend a Mass that is celebrated by a priest in union with the church of Rome. At present, as of January, 2013, reconciliation has not yet been achieved. The SSPX continues to be disobedient and defiant to the Vatican and the papacy. By attending the SSPX Mass, you would be participating in that disobedience and defiance.