-This was a fantastic episode for both Whittaker and especially Sacha Dhawan, who plays such an amazing Master. I don't know if that's the last we see of him as The Master, but he's been great in what I've seen here and in clips.
For Whittaker, though, she had an amazing final moments to save the day. And I really liked her send off. She didn't get a grand speech like previous Doctors, but instead, her "speech" was from her "fam," the group of previous companions. Which for her character, I thought was a great touch, because she was all about her "family."
She might not have gotten a big speech like Smith or Capaldi, or a near operatic send-off like Tennant, but I would argue her regeneration, in terms of visuals, was hands down the best. And FINALLY, a regenerating Doctor regenerates outside the TARDIS again.
-Loved the appearances of the previous Doctors. It was a nice touch, too, that when The Master became the Evil Doctor, much of what her wore was the costumes of the Doctors that appeared in the episode. I surprisingly got a little teary eyed when the Fifth and Seventh Doctors had a brief reunion with their respective companions. I didn't start watching Doctor Who until the 2005 reboot, but I've seen enough clips of the older stuff to appreciate those touching reunions.
-I barely know either of their characters, but I ship Ace and Graham so hard from their brief meeting.
-DAVID TENNANT. What? WHAT? WHAT!?