I feel like the first episode of two-part finales like this are really hard to review.
I don't fully know how I feel about the first episode's quality until the second part airs, as the quality of the setup is very reliant on the quality of the resolution.

But I did enjoy watching it. I loved all of the interactions between the characters, the rollercoaster of emotions that the Doctor goes on and Ncuti's performance. I thought the episode was visually really nice - I loved Sutekh's design and how they did the time window stuff. I think the episode's tension builds up nicely, as the Doctor is lured into a false sense of security almost, confident in his hope that Susan is his granddaughter, and confident that he can easily find out who Ruby's birth mum is, only for things to become more and more mysterious.

But this episode basically is all setup, and presents us with more questions than answers. We still don't know: who Ruby's birth mum is, who Susan Triad is (other than being connected with Sutekh), why Susan Triad was everywhere, who Mrs Flood is (which we might not find out next week), what Sutekh's plan is and, most importantly, whether or not Cherry Sunday is safe. This gives the last part a heap of stuff to resolve, which is something that I am a little nervous about, as I feel a lot of Doctor Who stories struggle to tie up all of their loose ends, despite starting with some interesting setups.

But yeah, I really think that I will only know how I feel about this episode until next week and I definitely need to rewatch Pyramids of Mars in preparation.