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" Fr. Z has also now weighed in on the topic.
It makes for a very interesting topic. Give it a look.