It's amazing if you haven't gone, especially when sunlight streams in through the windows. I saw a quote while I was there that was something along the lines of, "Gaudí is not an artist with whom posthumous collaboration is possible." A couple minutes walking around the church is all that's needed to confirm that notion.
I have been to Barcelona 4 times since I was a kid (1998, 2006, 2009, 2014), and every time the Sagrada Familia looked a little more finished, but still far from being completed. It blows my mind that it will finally be completed in 2 years.