Much has been written already about Sewell, and the two most recent reviews by Timonator and Clyde cover the experience well. Reviews from before 2018 show the development of the site over time but do not reflect the current situation anymore as both visitor management and conservation of the buildings have much improved.
Still, I have some practical info:
First and foremost: the CODELCO company that runs the area only allows access to people between the ages of 7 and 70. You have to state your age when booking – Fundacion Sewell does not check it but the CODELCO security people at the gate might.
You have to climb a lot of stairs during the tour. If you’re incapable of doing that, the bus trip out there might still be worth it and you can stay near the museum, but you miss out on a lot.
When you haven’t booked the included lunch, you can eat your own at the cafeteria. They also sell coffee and some snacks (and hotdogs).
Be prepared for a long and slow day. I took the option to start in Santiago (usually offered on Saturdays), we were requested to assemble at a spot near the Tobalaba metro station at 7.30. We arrived back there at 18.15. The tour is slow-paced and could be made more efficient by for example showing the safety video on the bus instead of the office etc. But overall it was similar to how other Chilean tours are run that I have been on. The tour leadership even was so laissez-faire that I lost the group for half an hour or so (or they lost me!). I waited for them at the museum where all tours end. One of the guides then gave me a private tour of the parts that I had missed including the bowling alley.
And some site trivia:
The brightly painted colours of the buildings in Sewell were a later addition (in the 1950s?), introduced by a painter who brought the idea from Valparaiso.
The former Industrial School (now the museum) was built after a modernist design.
Of course, this short review is also an excuse to show photos of two of the most remarkable indoor remains of the recreational culture at Sewell in its heydays: the top one is the swimming pool inside the ‘A’-class (American) social club, the bottom one is the bowling alley which was open to all.