I wasn't sure, when I first heard that Elisabeth Sladen would reprise her role as Sarah Jane Smith, and that K-9 would be returning to the Doctor's side as well, if this wouldn't result in the ultimate cheese, but no -- their return was handled brilliantly. I cried, cried I tell you, when K-9 sacrificed himself...and then cried again when he came back all shiny and new at the end. Never ever have I been so glad to hear the word 'Affirmative!' spoken in so affirmative and metallic-y a manner.
And for once, the story didn't seem rushed, and the plot didn't seem to be too full of dramatic twists and turns, nor did it seem to have any glaring plotholes in it -- and the acting was fantastic. ASH was absolutely brilliant guest starring as the evil Mr. Finch, and he seemed to be thoroughly enjoying himself, too. There was quite a lot of Uriah Heep and very little of Rupert Giles (unless it was his steely resolve and ruthlessness that came to the fore) about him, yet he was altogether believable and very, very sinister.
I loved the scenes between the Doctor and Sarah Jane - his excitement when they first meet, his response to her pain and recriminations, his pride in her achievements; it made him seem so human yet underlined the fact that he is, and always will be, the alien, the outsider, the lone wolf. Rose's jealousy and basic insecurity was also well delivered, and reminded me of Rose's reaction to Lynda-with-a-y in the previous series a bit. However, her relationship with the Doctor seems to have changed since then. Whatever they might have refused to call it, as far as I'm concerned there was a really deep, really close bond of love and trust between her and Nine, with a side order of UST coming from the both of them. With Ten however, I get the feeling that even though the bond is still there, the Doctor's feelings in particular are much less strong -- he seems to regard her more as a friend than as a significant other, while she still fancies the pants off of him. I could be totally wrong of course, but to me it seems as if he's purposely distancing himself from her a little more each week. And now, with allowing Mickey onto the TARDIS without consulting Rose, I don't know how that dynamic is going to work!
In short: I loved this episode, and will be glued to my television set for the next one on the strength of it.