It's true that's not easy to find, but if you have a map, and read the street signs, you cannot miss it.
I was really well treated, asked for vegan and they spoke very well english and were allways very polite and friendly.
Really huge porpotions of food, with soup, tea, dessert and more, for 4 euros you can eat and have not hunger for the rest of the day.
They even got other stuff and salted food if you want to pick up for later in the day.
It was a must in Praha for me because I was travelling with a very low budget.
Though the localization, you can access easily because there's metro or bus nearby, just look in the city map on this website like I did.
When I returned to this reastaurant with some friends, I even got more food and we all spent a lot of time trying to eat everything, and that was really good since we were going to travel to other cities.
Cons: Not in central Praha