Visited in May 2025. This site maintains a very high rating for a reason. The work is incredible.
One recommendation I hadn't seen on any of the reviews so far: read up on Antoni Gaudí and his non-traditional method of designing before visiting Barcelona! Learning of Gaudí's interesting life beforehand will enhance your experience and give you a perspective that the audio guide tours can help flesh out. (They will not give you the full story on their own.)
I think Kyle Magnuson said it best in his review here: "If all 8 Frank Lloyd Wright inscribed components were in one city, it might feel like this." Wandering around Barcelona, for as touristy as it is, is a treat as you search to discover all of the fascinating Gaudí buildings with their striking, non-traditional architecture style. Experiencing his work will make you lament how few buildings constructed in the modern day (especially in younger countries with few attractive historical edifices) have real personality and style, trading off beauty for price of construction & functionality instead. It's a shame, but viewing Gaudí's work inspires a sense of pursuing beauty for beauty's sake.
My wife and I have lived in Spain for eight months now and have literally seen more magnificent cathedrals than we can count. Every town has at least a handful, and they are almost always impressive. Despite this fact, the Sagrada Familia still blew me away. It's simply unlike anything else you can see in the world, and it instills a deep sense of awe and wonder.
Palau (Palace) Güell is a very interesting historical location and gives you a great look into the relationship and life of Gaudí's most intimate and longstanding patron. I recommend the tour here not only for the architecture but so that you can learn more about Güell himself, whose name is borne by so many of these inscribed locations.
We had no issues buying at-the-door tickets for Palau Güell, but as other reviewers have said, for some of the other locations like Park Güell, the Sagrada Família, and Casa Milà "La Pedrera" it is essential to book in advance.
A trip to Barcelona to see the works of Antoni Gaudí is 100% worth it, especially because you can visit Barcelona's second architecture WHS while you are there as well. You won't be disappointed.