I am fanatical about south Indian restaurants (idlis, dosas, the like). If you don't know what that cuisine is, see:
This is hard to find in continental Europe and recently I made a trip to Paris to see what I could find.
The most promising sounding restaurant I found on the net was Kannimaaraa in the Place d'Italie area, but this was either closed temporarily or closed for good.
After wandering about the Gare du Nord area where there are several South Indian shops and restaurants, we finally found a really, really good "Udupi" restaurant (purists note I use the term loosely to denote any 100% vegetarian south Indian restaurant that serves idlis and dosas).
The clientele at Krishna Bhavan is almost all madrasis (south Indians) although of what particular stripe I am not sure. The decor is not what you go to Udupi restaurants for and this one is no exception. The food was absolutely amazing. The best vadai I've had in years, a phenomenal and not too highly spiced thali, light and not too oily dosas. The idlis were good but not phenomenal; I have yet to find a restaurant that can serve a really good light and fluffy idli in quantity.
The restaurant is also very cheap, see the prices on their website (http://krishna-bhavan.eresto.net/). The thali is only 7.50 euros, if I remember correctly. We ate like absolute pigs and if I didn't live three and a half hours train journey away I would have been back there every day since.