Thanjai is pretty unique in Montreal since it serves South Indian food. And according to a Bengali friend, it's really authentic.
We visited the restaurant with a group of about 15 people. The service was not quite up to that: dishes got mixed up, one person waited for an hour just to get a glass, and in the end it took about 45 min to split the common bill. Overall the service did not leave a very professional impression.
The food was interesting, but a bit hit and miss. The dosas were great, but the Biryani and some of the deserts were eaten with considerably less enthusiasm. Since the restaurant specializes on dosa, I would recommend to stick with what is best!