Would love your opinion on whether it is worth the extra cost to get the more expensive Casa Battlo tickets that skip the line for a trip in mid September. Thanks!
I almost always buy advanced tickets but I failed to for our last September visit. We were able to get tickets the day before without a problem. The same for other Gaudi sites. There were many available tickets. Who knows if you would be as lucky? I don’t like long lines.
I don't see "Fast Pass" defined anywhere on the website; perhaps I'm just overlooking it. You still have to specify an entry time, so I guess it just means you are put at the beginning of the line of people sharing the same entry time. The regular tickets also skip the tickety-buying line.
The Fast Pass feature could get you in somewhat faster, it's true. They don't clear the building between groups, and it was my strong impression when I visited Casa Batllo in 2016 that the door-minder was letting X people in as X people exited. I suspect it's a safety (fire code) issue, because that place is mobbed. Being put at the beginning of the line for your time cohort could save some minutes. However, quarters inside are fairly tight and they sell a lot of tickets, so unless you opt for one of the very early-morning (higher cost) tickets, you will be in sardine territory no matter whether you spend an extra 5 or 10 minutes waiting outside the door.
Casa Mila/La Pedrera was also crowded in 2016, but it wasn't quite as bad as Casa Batllo.
Only you can decide how valuable your time is. Me, I have an automatic negative reaction to being asked to pay a substantial premium for a fast-track ticket after already having to tie myself to a specific entry time, so I simply wouldn't do it. It just feels scammy to me.
I see, however, that there are some additional features of the Gold ticket--the one including Fast Pass: you get to see the private apartment of the Batllos (don't know how interesting that is), and there are cancellation/change options not available with less expensive tickets. Perhaps those features have value to you.
Here's a blog post with a comparatively negative take on Casa Batllo: https://afternoonteareads.com/an-honest-review-is-casa-batllo-worth-paying-for/. The poster thinks Casa Mila is a better deal, because there's more house to see.
I enjoyed Casa Batllo in 2016 but was pretty shocked at how many people they packed in there. Tickets cost 22.50 euros at that time.