It's always good to plan ahead in general, but often you can get away with going somewhere 'on a whim'. This is not the case in Park Guell, Barcelona!
Some other plan fell through and I found myself with a few hours free in the afternoon in Barcelona. So I googled the address, and hopped on the #24 bus which took me "right to the entrance". So many things were wrong with this plan!
1) there are numerous entrances to Park Guell. The entrance I casually found with Google Maps was a 'satellite entrance', not the main entrance. This alone wasn't all bad, as this 'satellite entrance' was uphill from the main entrance, which meant my wanderings within the park were generally downhill.
2) Being a satellite entrance meant it wasn't a 'full service' entrance - the only way to buy tickets was online. No problem, I had my phone, but - the cell signal was useless at this location. No problem, the entrance had its own wifi signal, but my iPhone refused to connect to it due to some security concerns. No problem, there was a helpful park employee who somehow got me connected, but I could not navigate the purchase process (I'm an IT guy familiar with online services, but this was a real challenge). Eventually, after about 20 minutes of trying, I got my ticket and thus, gained entry.
3) There were no paper maps on offer at this entrance. No problem, they had a QR code for a electronic map. But this map must be the worst map ever produced; it was a funky 3-D presentation of the park, and I could not make head or tail of it (plus, it took forever to download over the poor connection). So I just basically wandered around and followed the signs that were sporadically placed around the park. After about an hour of wandering in the hot sun, I finally found myself at the main entrance, where most of the 'interesting' stuff is located. What a zoo! it was unbelievably crowded (and this was in late October).
So overall, I did enjoy seeing the park but I deeply regret not giving it more thought ahead of time, and not buying tickets in advance to avoid the nightmare experience I had trying to buy a ticket 'at the gate'.