I’ve been to La Bola twice in the last few years, once with Spanish friends and more recently alone eating at an hour that only gringo tourists eat at. The food on both occasions was great, the speciality is Madrid Stew, but the service was much worse when I was alone and several of the side dishes offered to the locals were not offered to me – I had to ask for the hot peppers, and they didn’t seem to happy at me setting the precedent for the other gringos.
So unless you speak fluent Spanish, go with locals, and go when they eat – the vibe is much better then. And don’t eat lunch.
La Bola, Madrid