As far as food quality goes, I guess it depends on what the standard Mexican restaurants are like in your area. Around here, standard Mexican means Tex Mex restaurant chains - Don Pablo's, On the Border, stuff like that. San Angel Inn is nothing like those. We thought the food was pretty good. Our main problem was the service, which was rather slow and not very attentive.
The main draw at San Angel Inn is the theming, which is wonderful.