Not much that I can say that others have not. This place is a breath of fresh air in Tokyo Station -- finally a restaurant where you don't have to worry about what you order. Ramen is the ultimate comfort food, but so much Ramen is in animal broth (ew).
This place is all vegan and it's all delicious. Their namesake ramen, T's Tantan, is my favorite. On a two week stint in Japan I ate here three times, I liked it so much. Service is incredibly fast, which I guess you would expect from a station restaurant. The atmosphere is quite relaxing though: the layout of the shop is designed to hide the station bustle from its patrons.
The only 'con' is that it is in Tokyo station, which can be quite busy. It takes a lot of people-dodging just to get there.Pros: Great food, Inexpensive, Calm inside
Cons: Tough to get to during rush hour