"6B" has two distinct ideas/plots, and the writers aren't quite able to reconcile the two, leaving me once the fence about the episode. The first is the old woman who sees a "ghost" of her husband, and it turns out her husband is actually from the other universe. It's rather sweet and advances the notion that emotion and love can linger across any boundary in a more scientific sense than just metaphysical. The second is end of the world, which is imminent because the old lady is causing a vortex to open up. This allows for major development between Olivia and Peter, which is what the writers were going for since the second season.
Coupled together, however, the two don't work as well as they would individually. That the end of the universe could be caused be a woman yearning for her husband takes away most of the gravitas of the end of the world and omits the other world entirely. The result is that the middle part of the episode is ginned up drama and not that compelling (though the morality stuff is interesting). In this respect, "6B" is a below average episode.
On the other hand, the scenes between Olivia and Peter are really terrific, with Olivia finally letting go in the end and allowing love to do its thing. While we don't learn anything about the machine or whether Peter was changed by it, the relationship between Olivia and Peter takes one step ahead, and the dilemma presented at the end of last week's episode, Fauxlivia's pregnancy, is amplified.
Score: 8.7/10