This is supposed to be one of the best places to watch flamenco. Unfortunately, the service, food and view were so bad (as well as the stage) that it took the enjoyment right out of the evening. Not only was the service mediocre, but they failed to serve me several of my courses. As shown on the picture, most of the time your view of the show is obscured.
Almost without exception, all the restaurants in the Plaza Mayor area are tourist traps, created exclusively for the entertainment of the guiris. You may be wondering what a guiri exactly is; well, it is the Spanish slang word for "totally lost-sun burned-wearing socks and sandals tourist". ;-) Not a pejorative adjective, but certainly not a positive one...
Unique Suggestions: If you are really dying to try the tacky and overpriced "typical spanish" experience (fake flamenco dancers, fake bullfighters, but thanks god real sangria), I'll say you better go to "Las Cuevas de Luis Candelas" (in the picture) at Calle Cuchilleros, 1. The atmosphere is, at least, interesting.
Fun Alternatives: Although Madrid is not the best place in Spain for Flamenco music, I'll totally recommend a visit to Candela, in Calle Olmo, 2. There you could witness wonderful live Flamenco performance while drinking wine (probably more than one glass) ;-) with the local crowd.
There are many places which are said to hold good flamenco. Hesitate it. Good flamenco is rare and you can find in very specific places. Do not pay for a tourist spectacle. It´s not worth.
If you take a taxi be cautious because sometimes the driver uses to give some rounds in order to incremente the fianl price.