We arrived quite early as Nirvanam has a reputation for being especially crowded at lunch. We were invited to start straight away, which we accepted at a pace. The variation of curries and sides were good, and we began with a combination of dishes. The curry choices were from mutton masala, sambar (lentils), dal palak (spinach), and bhindi masala (okra). The starters and sides included andhra chilli chicken, chicken biryani (made with basmati), paratha, onion bonda (bhaji), and some salad and Japanese rice. I went straight for the mutton masala, closely followed by some bhindi. The mutton masala was excellent, and I would probably come back for an evening meal based on that alone. The bhindi didn't quite hit the mark (not a massive fan of okra), though the sauce and quality of ingredients were good. The paratha, though not quite as nice as it is when served immediately, hit the spot. The bonda were a great little addition. We polished off a fair few of those.
On the second visit, we sampled the biryani and the andhra. The biryani was nice, though contained a bit too much chicken skin, and the andhra hit the spot - quite a bit nicer than it looked (all looked a bit grey sitting there). The andhra definitely made up for the fact there was only one meat curry (the Raj was so good for this). The sambar and palak were just ok, but I wouldn't want to judge completely on general curry buffet fare.
Based on the food, I would go back to Nirvanam, though I wouldn't travel as far as I had to just for the buffet. If it's not too far to travel, I would recommend it (there were hoards of people waiting by the time we left, so they must be doing something right). Go there early if you want to get into the buffet action straight away, and don't feel the need to be rushed even if you get a the concerned look from the waiter that you're going for a third full plate.