Awesome tour! The Carrie Furnaces were the highlight of the trip for my out-of-town guests this summer. It's probably the most uniquely Pittsburgh thing you can do here as a visitor. I don't know any place else where you can get a tour of a former blast furnace site and learn so much Pittsburgh steel history and steel science in just two hours!
The tour guide was very knowledgeable and he admitted to things he wasn't sure of. Apparently you learn a lot of different material just by having different guides!
The location is a lot easier to get to, now that there is a paved road off of the bridge. When I came down for a yoga + beer blowout last year, we took a dirt road and weren't sure if we were in the right spot. Now it's pretty clear where you are going and it was also pretty quick to get a uber back from the furnaces.
Just a warning, there is a fair bit of walking and stair climbing, for those with bad knees. It's doable if you take it slowly, but keep in mind that it's an old industrial site.