Having been to this place more than fifty times, I will write this review in comparison to the other South Indian place Madras Cafe also present on King's Circle. Both the places of course have similar family origins in the Ramnayak family (Founder of the first Udipi restaurant in the world).
Where Mysore Cafe scores:
1) Sambar: The Sambar of Mysore Cafe is much more authentic and tastier compared to Madras Cafe
2) Idlis: Scores in both quality and variety. Made in a traditional manner, you also find jackfruit (khoto) idli as well as idli gadbad
3) Service: As many people have mentioned earlier, Mysore Cafe is still much better in terms of service to Madras cafe. Atleast the waiter will take your order fully and properly without running half the time.
4) A/C: Mysore cafe also has an ac section and also a full meal annexe. So if you don't want to sweat it out at Madras Cafe, head the Mysore way
5) Specials: Mysore cafe comes out with specials like aamras puri (highly recommended in summer), most specials (if any) at Madras Cafe get over by noon or early evening.
6) Open on all days: As opposed to Madras Cafe which is closed on monday, Mysore Cafe is open on all days.
7) No waiting: This is the knockout punch. You have the choice of waiting for 10-15 minutes outside Madras cafe or getting a table almost instantaneously at Mysore cafe
8) Celebrity Endorsements: On a lighter note, who can disagree with one of the smartest and richest guys in India, Mukesh Ambani about the best South Indian restaurant in Mumbai? That is Mysore Cafe for you
Where Madras Cafe scores:
1) White butter idli. Absolutely marvelous, don't even think of having your idli with yellow butter
2) Cozier ambiance. Being smaller in size, Madras Cafe gives a more cozy feeling than Mysore Cafe
3) Better management: I see the two sons of the owner really trying to improve the place. Service has improved and I even have seen owners apologizing and replacing food in case it is not up to the visitor's liking.
4) Coconut Chutney: Made the authentic way as opposed to some green vegetable mix chutney at Mysore
On a concluding note, I think both the places have their strong points but a 15 min. waiting queue outside Madras cafe and a half empty Mysore cafe is just pure market inefficiency. People please give your love to both the places.