That is the question. This question arises from a letter sent to parishioners pre-motu by Fr. Jay Scott Newman in South Carolina in which he stated that it is sinful to attend a mass at an SSPX chapel. Brian Mershon has addressed the issues at length in a piece entitled “SSPX in schism? You can believe Fr. Newman… or you can believe the Church” Some excerpts:

In his bulletin letter, Fr. Newman wrote, “For now, however, I write to warn you about a group of renegade bishops and priests who are leading people out of full communion with the Catholic Church in the name of the old liturgy.”

Compare Fr. Newman’s description of the SSPX bishops and priests as “renegade” with the term Darío Cardinal Castrillón, Prefect of the Pontifical Commission Ecclesia Dei (PCED), used at the May 16, 2007 Conference of Latin American bishops where he explained how the PCED was fully engaged with bringing the SSPX bishops and priests into full canonical regularization. Cardinal Castrillón calls them “brothers.”
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While the PCED confirms that Archbishop Lefebvre and the four consecrated bishops excommunicated themselves when the 1988 ordinations took place, it repeatedly stated that the SSPX priests, while suspended, are not excommunicated, nor schismatic. Despite Fr. Newman’s warnings to the contrary, the lay faithful incur no sin, nor any penalties whatsoever. The SSPX priests, of course, rely upon a canon in the Code of Canon Law which speaks of “a state of emergency within the Church,” and that “the salvation of souls” is the ultimate goal of Canon Law.

Fr. Z has also now weighed in on the topic.

It makes for a very interesting reading. Give it a look.