Some caveats: this was maybe the second Dr Who episode I've ever seen. Trek was the only media fandom I was into as a kid. The nostalgia stuff about the dog went over my head, though the shape of the story was clear. The previous female companion story was also comprehensible to a non-fan, though it lacked the emotional punch with me that it probably had for a long-time fan.
Let's get the important question out of the way first. Anthony Head can do creepy extremely well. The simple act of greasing his hair straight back turns that face menacing. He's all cheekbone and jaw to start with, and his face turns harsh without anything softening it. His voice is breathy and seductive in the scene where he's attempting to talk the Doctor around to his side. I had a flash of thinking that it's hard for Head to come across as truly evil to me, because that voice feels tender. But then, that makes for some great evil.
The script was utter crap. It made no sense at all. Classic SFnal premise at the core: being wishes to gain godlike power to restore past wrongs, fight entropy. Good guy refuses, because death and change are essential. (Man, I so want to write a story in which this is just not true. Death is the ultimate horror, utterly evil and wrong, and whatever you do to stop it is heroic.) Tragically this familiar idea got about 30 seconds of development. And the story was at odds with itself. The alien race was supposed to welcome change, to be open to adapting the physical and mental characteristics of races they conquered. Finch's argument to the Doctor was an argument for stasis, however.
What was Mr Finch's actual motive? His bat brothers were into eating students, I guess, but Finch himself in the pool scene seemed to have some interesting things going on. This all vanished at the end when the conflict devolved into "eat them all". If the Doctor thought the aliens were just baddies and would use their god-powers for evil, then there should be no conflict; just stop them. He can only be tempted, I think, if he believes Finch himself is well-intentioned.
Heaped up around this basic story is a bunch of skiffy muddle. Students at a high school are being controlled by something weird added to their french fries. This substance also turns them into high-powered calculators. Not all of them, though. Except when it is all of them, when they sit at terminals watching a crappy repetitive green graphics sequence, representing their input into an attempt to crack some universal code. The teachers were all evil non-intelligent bats who got lots and lots of screen time swooping across an allegedly creepy full moon.
What this script needed was somebody ripping away all the extra crap to focus on the main theme.
The K9 sacrifice (so Floydian, so heavily underlined by the script) was cheapened by the restoration at the end. But I guess that was a sentimental thing for the fans.
In short, enjoyed ASH, don't feel any need to watch Dr Who again.
ASH was given a couple of good lines:
"Forget the shooty dog thing."
"You bad dog."