The food at San Angel Inn was fabulous when I was there. What you have to remember is IT IS NOT TACO BELL. San Angle Inn is more authentic mexican food, not tex-mex, not fast food mexican. If you go in expecting tacos, you'll be disappointed, do yourself a favor look at a menu and see if you like anything, if not go with Biergarden, if it looks good go with San Angel.
I asked my waitress between two steaks (and I don't really like steak) she told me to get the Filete Montuleno. It was the best steak I've ever had, literally melt in my mouth, sauce was great, plantains were awesome it was an all around great meal. I had the bread pudding for dessert, I don't think they have that anymore.
Enjoy yourself whatever you have for dinner