I stayed at Arien International 3 times. It's located on a quiet street in Kalkaji, South-Delhi. The best thing is that there is hardly any traffic on that street, so I could sleep well - a rare experience in Delhi. The rooms were really clean. The receptionists proved to be reliable. The rooms are strangely better than on the pictures.
It's a bit expensive for me (2500 Rs. double room in year 2011), but the good sleep and the clean, comfortable bed worth it after a long flight.
Plus, there is a superb pure-vegetarian restaurant nearby (5 minutes walk) called Naivedyam. Ask for directions, it's worth trying.