There are two part to the park - one is publicly available and there is 'Monumental Zone' for which you need tickets. Often you need to reserve them few days in advance - so don't count on just showing up, buying a ticket and entering.
The park is vast and beautiful, perfect for a stroll. As it's on the hill you also have beautiful panorama over Barcelona.
The monumental zone is almost fairly-tale like.
So beautiful, such nice staff, so accommodating, we even showed up super late and they let us in! They have a restaurant in the park and we grabbed food for breakfast and it was delicious, and not over priced. We purchased the 8:30 tickets and that was a great idea if you want to take pictures without other people in the background! It starts getting crowded around 9:30-10:00, but it's great if you're a solo traveler because you can ask people to take photos of you! A great experience!!!
Visited here and thought this was one of my favorite places. All of the mosaic detail is beautiful and there is a spot at the top with a bunch of curved benches that is amazing for pictures. There is a limited amount of tickets sold for a time period, and I didn't know that when I first got there in the morning. I ended up having to buy my ticket for in the early evening, which was fine, but it would have been nice to know before going all the way out there.
It was amazing but I definitely recommend booking ahead as the lines to get into the paid part of the park can be quite long. They allocate a time to go. I was there in May and even that was quite hot so bring water and a fan! The park was cool though and it had a great view of Barcelona and the harbour. There are a number of hawkers selling their goods, so bring small change if you would like any tourist items.
...details of my own experience. We got there without checking online first, (our fault here) and to enter the monument we should have waited for 4 hours. So my suggestion is to check online first and plan in advance. The place is always crowded. And for good reason. It is absolutely wonderful. An amazing example of what a brilliant creative mind can do. I highly recommend to add it to your list of you intend to visit Barcelona... But plan ahead!! ;)