If this was anyone's first episode of Star Trek, I wouldn't blame them for never wanting to watch it again. There's no sugar-coating: this is a terrible episode.
Any hope of continuing Trek's penchant for taking possibly terrible/cheesy/not compelling concepts and elevating them beyond their obvious potential is thrown completely out the window. I'm pretty confident that this is worst possible telling of the concept at hand. Just what is the concept at hand? Ship encounters a man trapped between universes. If his two selves meet (matter and anti-matter) both universes will cease to exist.
Rather than just deal with the conceptual ramifications of the subject matter, the stakes are raised in an arbitrary and unsupported way. Regardless of the scientific accuracy of the setup (which must be wonky at best) it is so poorly set-up and so sloppily executed that the unbelievable stakes carry no weight. The story points on which the drama rests are treated with no consideration as to how it is revealed to the audience. And the drama is pretty meager in the first place.
It's unfortunate to see such ineffective tactics used to heighten suspense, when the show has so readily proved it's ability to raise them legitimately and most invisibly.
Any character bits to save the episode? No. Any exciting action or suspenseful sequences to wash out the bad taste? No. Are the performances at least acceptable? No! This is a perfect counter-point for my belief that Shatner is at his best when the dialogue is already great on the page ... he takes the text and just knocks it out of the park. Here? It's just maddeningly, atrociously not up to the standard this show has set for itself.