A spoiler cut, for those of you who are a week or so behind on 'Doctor Who'...
I have to say, that really was an excellent Borg story. Much better than the Next Gen/Doctor Who crossover they did in the comics. This really used the Borg well--the whole "upgrading" thing made them into a genuinely scary and unstoppable menace (as opposed to the way they were used in Next Gen, where they always "adapt" at the beginning of each episode, then forget all the adapting they've done by the next time they run into the Enterprise. "Oh, hey, phasers! Now how did that trick for being invincible to phasers go? Oh, bugger. Should really leave that on all the time, shouldn't I?") The Doctor's look when he was partially assimilated really captured the Borg aesthetic, and I thought it was clever that the Borg Doctor was tricked into immobilizing the entire Collective by forcing them to devote their resources to an unsolvable chess puzzle. On the whole, this was quite possibly the best Borg story ever, even better than "Best of Both Worlds".
A pity it was intended to be a Cybermen story instead.
Look, I'm not going to pretend for a moment that I didn't have fun watching it. It was glorious fun watching Matt Smith ham it up as Evil Matt Smith, even if it didn't have a hope in hell of making any sense in light of the Cybermen's avowed ethos of "logic triumphs over all". Warwick Davis is absolutely wonderfully naturalistic, tossing aside the twee japery of Professor Flitwick for a world-weary, sorrowful man with the weight of the universe on his shoulders. Clara was great, because Clara is always great. The Cybermites were creepy and icky and cool and well-realized. It's just...
The thing about the Cybermen isn't that they're powerful. It isn't that they're fast. It isn't that they evolve and adapt to eliminate their weaknesses (which they never did, in fact, until this episode. They kept the sodding gold weakness for almost two decades, for bleep's sake!) The thing about the Cybermen that make them interesting and unique is that they are doing the absolute smartest thing for them to do under the circumstances. That's what nobody ever gets right about them, with the exception of Marc Platt. It is that if you look at it rationally, the Cybermen are absolutely right and doing exactly what they should do under the circumstances.
After all, they are smarter than humans. They're tougher, faster, stronger, less vulnerable to disease or injury. They live longer, they can be fixed more easily...the only "disadvantage" they have is that they're not irrational. They don't panic, they don't grieve, they don't engage in self-destructive behaviour or get depressed or spend all their time involved in overly complex reproductive rituals. But the very fact that we see irrationality as an advantage is, itself, irrational. Once you lose your emotions, you realize that the need for emotions was, in fact, an emotional need. Viewed through the lens of pure logic, Cyber-conversion is the only way to go.
'Spare Parts', which remains the gold standard for the Cybermen (er, wait...that'd be bad, wouldn't it? Oh, well. The platinum standard then) gets this perfectly. The Mondasians are in dire straits. Cyber-conversion is the only way to save their species, and the Doctor's insistence that they're dooming themselves to a living death as emotionless monsters falls on deaf ears because a living death sounds better than a dying one. Becoming a Cyberman, becoming a race of Cybermen, is brought home as a choice and not just something inflicted on you by the monsters.
That's what the true tragedy, the true fascination of the Cybermen is, and that's what Gaiman didn't get even as he wrote a wonderfully entertaining episode. The Cybermen chose to be what they are. They really do think--worse, they really do know--that they're doing the right thing. They are evangelists of pure logic, preaching the gospel of posthumanity to all us heathens who remain attached to fear and pain and sorrow. Next to that, shouty Matt Smith isn't even a distant second.