This place was on a bunch of lists, but sounded like it could have been over-touristy, given its…read morelocations and being on so many touristy "must see" lists - it is absolutely not, it is extremely legit, I'd come here all the time if I lived in Madrid.
The calamari you're probably here for, isn't literally the best calamari on the planet, it's just fried calamari that you'd get anywhere, but it is absolutely great calamari, perfectly fried and seasoned, and you get a *lot* of it. Since everything was so affordable and I'd heard such great things about the calamari, I ended up ordering (for me and my wife) both a calamari sandwich and a cup of calamari, also a sausage sandwich and a cup of patatas bravas. From this, I learned, if I were ever back in Madrid, I'd definitely go back here and get three of those things again, by which I mean, yes, they stuff the sandwich full of calamari, but that's still nowhere near as much as the absolutely absurd amount of calamari they stuff the *"raciones" (i.e. just a cup of calamari) with, so that ended up being way too much food. The calamari was obviously the star, but the sausage was also pretty tasty, and that sandwich didn't fall apart nearly as much as the calamari sandwich did... I ended up stuffing the sausage sandwich with some additional calamari from the overflowing calamari sandwich, and that is absolutely the way to do it: get a sausage sandwich, a cup of calamari, and stuff your own combo sandwich. Or just skip the bread entirely, honestly. The potatoes were also fantastic, and you also get an equally absurd amount in a "ration". One note, though, the potatoes come pre-sauced in a portion - the calamari, by default, do not, so you absolutely do want to pay the .40 euro extra for the sauce and some extra lemons, as it absolutley does need both for the full effect.
Another note - as I expected from other reviews, yes, there was a huge line when we got there. This line is, notably, *only* to wait for a table, which makes sense, as the restaurant itself isn't very large. There was a separate line for to-go orders, which had exactly one person waiting when we got there, so there was about a 30 second wait to order to-go. So if you're staying nearby, that is definitely the way to go. They also don't have a lot of drink choices, so I would strongly recommend what we did, bought a couple bottled ciders from a grocery store on the way and had them with our food back at our hotel.