Previously I did mention to you how although the idea that Pius XII was the last pope was the most common position among sedevacantists, it was hardly the only one, with some claiming he, too, was an invalid pope, and thus you end up with some going back further, in some cases MUCH further (the farthest I ever saw anyone assert it was that the seat fell vacant in the 10th century). And then you have some who are willing to accept some of the popes after him as being valid popes. There's a lack of agreement, which ends up showing how arbitrary it is.
To answer your question, though, I've never been sede.
My opinion on sedevacantism is that it's just Protestantism except without the intellectual honesty of Protestantism. It preaches that anyone can just decide on their own opinion that the pope is a heretic and therefore not pope. This is a position that makes perfect sense in Protestantism, where everyone exercising their private judgment on what's valid doctrine is a feature rather than a flaw--heck, Protestantism developed because of people concluding the pope was (in their view) a heretic and that popes had been heretical for so long that they should leave the church--but is incoherent from a Catholic perspective.