Apart from the soft idlis as mentioned by Ranjani, I wanted to add the other aspects of Murugan Idli as well.
Murugan Idli serves nice chutnies with their idlies as well. Coconut, Pudina, Onion and Tomato are the ones served apart from the traditional Chutney Powder (Molagai Podi) with Oil.
And it is not only idlis that are served, rather Dosas and Vada are also served in Murugan idli and that is all about the menu.
The major minus point at Murugan Idli outlets are that due to their popularity, the waiting time at the restaurants is pretty long and at times gets on the nerves. This is during the popular timings such as Afternoons and Evenings. If you are a person who values time more than the taste of the food, please visit the outlets only at non-peak hours.
Trust me, I am from Chennai and have lots of first hand experiences at this place.