The noodles were... fine.
What I learned over the next four days is 1. the food at Xi'an Famous Foods in New York is better than most of the food in X'ian itself and 2. everyone tells you to eat in the Muslim Quarter but the best food in Xi'an is not in the Muslim Quarter. In fact, my favourite meal there was from a nameless shop recommended here. I know nobody wants to read another piece of food writing about simplicity and authenticity and letting the ingredients speak for themselves and feeling a real connection to somebody's heritage – but I'm afraid this was inescapably one of those.
Finally, while you're mixing the sauce into the noodles at your table, he brings over a soup bowl full of water from the pot, which is the same water he's been cooking noodles in all day, so it's essentially a kind of wheat broth. That's the whole meal, and it's satisfying beyond belief.
Now, I would never normally have gone into this shop, because it was cold and grubby and there was only one other customer. My first rule of eating in foreign countries is not "Head straight for restaurants that have a vaguely depressed quality." But all of my expertise disintegrated upon contact with China. Which is the only reason I was even willing to contemplate signing up for an organised street food tour after I arrived in Sichuan Province. I was glad I did, though, because the Chengdu Food Tour turned out to be a really good use of an evening, climaxing with the famous pig brain mapo tofu at Ming Ting.