This is an Unesco heritage site because it is still (almost) in the same condition at the last day of its operation.
You should take the guided tour, you get so much good information during the 1-hour tour. The tours are organized every 30 minutes. Almost all of the tours are in Finnish but there is a 10min film before the tour which has English subtitles and I believe you can have an English summary (leaflet?) from the info point.
The Verla is in the middle of nowhere. However, the surroundings and the setup are beautiful.
If you are travelling close to Lahti, Kouvola or Mikkeli, this is must place to visit. The smaller roads are also very pleasant to drive across the Finnish countryside.