One of the delights of this episode has to be the growing ménage a trois evident between Spike, Drusilla and Angel. Dru's vindictive torturing of Angel reveals a lot of back-story, and perfectly sets up the episode's coda, where Drusilla rises from the wreckage of the showdown with the Slayers, her strength - if not her sanity - restored.
The contrast between Kendra and Buffy, who have radically different approaches to their role as Slayer, is also nicely realised, especially when Buffy has to admit to and justify her relationship with Angel. The differences between Buffy and Kendra serve to more closely define Buffy's ethics and methods too. They also provide some clues about how Giles and Buffy's relationship might have developed under different circumstances.
The arrival of Kendra is, nevertheless, something of a distraction, and temporarily collapses the momentum of the story. It also serves to punish inattentive viewers, who wouldn't remember that, since Kendra is not one of the three assassins, there was still one assassin unaccounted for!
There's a palpable feel of déjà vu about the basic plot outline (the vampires summon help to destroy the Slayer, who is interfering in their plans for world domination). It's becoming a recognisable formula for too many episodes. With the luxury of two episodes at their disposal, Gordon and Noxon should perhaps have delivered something rather more ambitious and substantial.
Xander and Cordelia's blossoming relationship is a welcome addition, correcting an imbalance in the Scooby gang's group dynamic created when Buffy crushed Xander's hopes that their friendship would lead to a more adult relationship.