First things first, it's Q3 2021 and the tunnel is open again. With it the site is fully accessible for visitors. I had waited for more than a year to complete Poland again; Covid travel restrictions may have also played a role.
The tunnel links multiple mines and follows the flint stone layer upwards, giving you a good understanding of the spread and rather limited depth of flint stones. It's assumed that the flint stones formed as a ground layer of a lake, so the flint stone layer tilt upwards. Supposedly, the deeper layers are of better quality. Side note: Flint stone mining was a temporary activitiy. The soil of the area was too bad for permanent habitation.
While technically, you can tick the site by roaming around the premises or the other locations (many open pits remain and are visible), you would be missing out on the key feature of the site: The small painting of a woman giving birth (?!). I am not sold on the giving birth part, but it sure was nice to see down in the tunnel.
In comparison to Spiennes which gives you an idea of the narrowness of a flint stone mine, Krzemionki feels more explorable and gives you a better overview, especially in combination with the onsite museum, consisting of a multi media part and an outdoor museeum.
Getting There
I came by car with a friend, but had investigated public transport options as a fallback. The main town with connections across Poland (some direct trains, and more direct busses) is Ostrowiec Świętokrzyski. The main train line between Warsaw and Cracow runs further west past Skarżysko-Kamienna and Kielce. Google maps has some weird connections, and you should also consult en.e-podroznik.pl.
From Ostrowiec Świętokrzyski you can catch a bus the visitor center every 2h (according to google). Or walk in roughly the same time you would have to wait for the bus.
Getting In
You can reserve online (and I had a reservation). But it's not too strict. We came two hours early, one more person than reserved and it wasn't a big issue to change the tickets. Only problem was that the tour was in Polish, but luckily my friend did the translation.
I think overall they try to provide some context and use reconstructions to make the experience more tangible than e.g. Spiennes. No objections here. Having a better offer for non Polish visitors would have been nice. And the multimedia exposition was a bit over the top. Unfortunately, it was the first part of our visit, so we couldn't simply skip it.
While You Are There
The site is in the middle of nowhere. I think this will always be a stopover moving from Warsaw to Cracow.