leonard nimoy is rather stiff and even somewhat dull in comparison to martin landau, though certainly competent.
the loss of barbara bain's cinnamon is significant. she added a flair and sly smile.
bain's award winning performances coupled with landau's prodigous acting talents and versatility are sorely missed. the show doesn't have quite the same feel as it did before (and never got it back).
that said, this two part episode is on par with the some of the best episodes of season 3. definitely worth watching.