Most European attractions require advance booking, but buying tickets online is very convenient, which is great for someone like me who decides to visit on a whim while on a business trip.
As for the Sagrada Familia, it's fine if you see it as a work of art by Gaudí, but as a church, it feels a bit ostentatious.
In short, it's a landmark building, and you definitely shouldn't go in to see it, but if you've seen the Sagrada Familia in Cologne, Florence, and the Vatican, this one is just so-so.