I have never had the fortune (whether good or bad, according to some) to see this structure.
But I have learned a great deal of it through the art critic Robert Hughes' "Shock of the New" BBC television series from the 1970s and his eponymous book on "Barcelona".
The cathedral is amazingly complex and organic, heaving spires into space, and also enclosing space with curves teeming with forms.
Its design broke down and reconstituted the image of a Western cathedral into the Catalan influences of Antonio Gaudi's personal history and the era of modernism in which he lived. This included influences from Art Deco, Dadaism, Futurism and so on. But Gaudi never fell into any particular school of art, was a truant to them all, and pursued a vision so strange that it stands there, unfinished yet somehow complete.