We went there for takeout after reading so many nice reviews, and found Nini's to be every bit as good as people say. Knowing it might be our first and last time eating there, we (a couple) ordered a large selection of items, including veg quesadilla, soft corn tacos (veg and fish), and a variety of sides (guac, veg black beans, rice, two types of salsa).
The tacos were DELICIOUS. Nini's tops their tacos with a spicy, vinegar-based slaw that adds a nice burst of refreshing acidity that differs from so many "heavy" feeling tacos. The guacamole was also a winner, as was the mango and habanero salsa. In fact, the only item amongst everything we got that we didn't love was the rice, which was frankly dried out and kind of flavorless. Zia Taqueria has much better lime-cilantro rice!
Rice aside, the food was great and the counter service was brisk, efficient, and pleasant. Everything we got, and it was a lot, came in at ~$35; a couple could easily eat a great meal there for about fifteen bucks.
I would recommend Nini's unconditionally to anyone visiting Durango.