It helps that Philip Madoc is so delightfully ambitious as Doctor Mehendri Solon, the would-be Dr. Frankenstein of this story. He's apparently a brilliant neurosurgeon, but what makes things so entertaining is his fixation on things like the Doctor's head. There's a purpose behind it, of course -- he wants the Doctor's head to complete the pieced-together body he's built -- but the way Madoc plays it suggests that he would be interested in some head-stealing even if he wasn't trying to give his master Morbius a body again. Solon also gets all the best lines, such as calling his assistant Condo a "chicken-brained biological disaster." But Colin Fay as Condo is also doing a fabulous job, playing Condo as entertainingly dim but not going too far with it. As a result he's quite a sympathetic character, even if he's on the side of the villain and keeps doing villainous things.
The Sisterhood of Karn, on the other hand, isn't quite as well realized -- the sense of mysticism surrounding their actions and abilities is a good move, but there's still a sense that these are something of a backwards people, despite their relationship with the Time Lords. It doesn't help that they're one of these groups who've decided ahead of time that the Doctor must be guilty of whatever plot they're concerned about and thus don't give him a chance to explain himself. Still, they have the power to mentally transport both the TARDIS and the Doctor to their shrine, and to make Sarah go blind, so they're clearly not a group to trifle with. How you feel about their elaborate dances and movements is probably a matter of taste (I personally can't quite decide if they're very silly or wonderful).
But where these two episodes excel (for a more dedicated fan like me, at least) is in their development of the mythos of the Time Lords. Most of it is (shrewdly) kept vague, with talk of alliances with Time Lords and special healing elixirs and (most excitingly) a Time Lord criminal named Morbius who met his end on Karn. Obviously the Morbius bit is the main thrust of the story, but the circumstances behind his initial downfall are kept pleasingly indistinct -- it's enough to know that he was a villainous Time Lord who met his end on this planet. It's also a nice twist in the cliffhanger to part two, with a blinded Sarah stumbling down into Solon's lab after hearing Morbius calling, and the audience learns that Morbius, this famous, deadly enemy of both the Time Lords and the Sisterhood, is now literally just a brain in a jar. Good stuff.
92 The story goes that Dicks, unhappy with the rewrites, requested that his name be taken off and replaced with "some bland pseudonym". When he saw the name Robert Holmes had chosen, all was, it seems, forgiven.