Magical! I've seen plenty of churches all over the world and there is literally nothing like the Sagrada Familia anywhere else. Seeing Gaudi's vision come to life is an amazing experience. You could spend all day here and still miss a thousand tiny details. I suggest taking your time and don't let the guided audio tour rush you through! Definitely book your tickets ahead of time - I debated buying tickets online and simply forgot, arrived in Barcelona on a Sunday and when I tried to buy tickets, the first available date was for Thursday (and I was leaving on Friday!).