Finally made it to Machu Picchu in Union Square tonight, after months of abortive plans. And after really looking forward to it, I was a little disappointed in the quality of the food. (I can't speak to authenticity and so I won't try.)
I ordered the lomo saltado, recommended here and in reviews. It's kind of a stir-fry of sliced beef, tomatoes, red onions, and french fries, tossed in a very scant and tasty sauce with some cilantro, and served with white rice. The beef was extremely tough, likely overcooked (perhaps cooked twice?), and both the beef and the rice were less than hot.
My wife ordered the fried rice with chicken, which was generally good, but had way too much soy sauce for my tastes.
Service was friendly; the entrees arrived separately, at least five minutes apart, and generally took a long time to reach the table.
I'll go back at least one more time to sample the ceviche, but I'm not sure that I'll go back for anything else.