I think its hard to pinpoint "authentic" Mexican food. And I do know that many of us live in the southwest and have lots of family owned, mom and pop restaurants and San Angel may very well not taste like any of them. However, Mexico is a large country, with lots of regions so it would go to reason that there are various forms of authentic Mexian cuisine. Just as there are many forms of American food from fried okra in Texas to clam chowder in New England. Is one absolutely more authentic than the other one? San Angel may be as authentic as Taco Bell or it may be the real deal of some region in Mexico not commonly found in American based Mexican restaurants. No idea but we have enjoyed many a great meal there and never had anything but fine service. It is definitely not Tex Mex and Tex Mex (my personal fave, I am from Texas after all) is not the only style of Mexican to be found and therefore not the last word on what is true Mexican food. I live in Colorado now and most people here find Tex Mex to taste like a tv dinner which is why the vast majority of Tex Mex restaurants have gone by the wayside. Too bad, I love Tex Mex! But I think San Angel is also good, just in a different sort of way.
I have to chuckle, this restaurant gets more people worked up over how much they hate it than any place else! I once got called a moron by someone who just was convinced I had to be one to like this place.
I always say give anything a try, your taste buds are just that, yours!