I didn't care for Matt Smith's cameo. A redundant passing the torch which has already been passed and actually ruined the momentum of the new Doctor. So we have Capaldi, in costume with sort of new console room, essentially declaring "I'm here, I'm the Doctor now." And then we get Smith popping back to say all those things that should be said in the regeneration scene and were. Redundant and pointless.
I don't know, I think we needed that scene in order to get that great moment of vulnerability from Capaldi's Doctor at the end, as he feels like Clara just can't see that he's really the same guy she just talked to on the phone.
Not only does it make you instantly feel for him and see that he really
does need Clara's help and acceptance, but it makes you understand why Clara would want to stay with someone as outwardly cold and abrasive as Capaldi in the
first place.
Because otherwise I'm not sure I really would have bought her wanting to stay (especially when she's not obligated to follow him around like companions of the past have been).