I stayed at BnB Mi Casa for the entire summer of 2011 (I was working and doing research in Delhi for three months). I also just went back to BnB Mi Casa in March. The BnB was a great respite from the hot and busy environment of Delhi. The staff there is very helpful and willing to go above and beyond to help, like allowing me to store my luggage at the BNB while I traveled elsewhere in India, or even making copies of reference books for me so that I could take them back to the US for my research. The staff is also very good at cooking. The breakfast is very good, and they make a very good Indian dinner. I discovered that they are also very good at making a basic pasta with red-sauce, which is often what my stomach needed after weeks of eating spicy Indian food.
Also great about the BnB is that it is very close (2 min walk) to one of the main metro lines in the city, meaning it is easy to get anywhere in Delhi. In addition, the BnB is right next to a park where I would often go run or walk.
Overall, I can't recommend the BnB highly enough as a place to stay in Delhi.