A proper visit to this WHS should consist of two parts: to the Messel Pit itself and to a museum where the fossils discovered there can be seen. So my day started in Darmstadt, at the Landesmuseum. There they have two (small) exhibitions on the Messel fossils. One with the bigger animals (like a crocodile) and one with the insects and other tiny animals. Especially on these, you can still see the colouring which is so remarkable about the Messel findings.
After lunch, I drove to the Pit itself. There are signs to Messel from (parts of) Darmstadt: it's about 10 km. For the guided tour about 50 to 60 people had gathered at the small info center. We even had to be split into groups: a remarkable large crowd for such an inconspicuous place. I had thought that only the WHS-addicts would make it here (a bit like the Neolithic Flint Mines in Belgium).
The story of the guides absolutely makes a visit worthwhile: without it I wouldn't advise going (the viewing platform doesn't tell you much about what has happened here). Only that way you can imagine the lake, its volcanic origins, the wild animals (even including a tapir) swimming or drinking, the rain forest, the Mediterranean climate. And the special condition of the oil-shale lake bed made it possible to conserve the fossils so well.
The Pit's more recent history, from not-too-productive mine to (proposed) garbage dump to WHS and recovering biosphere, is an interesting story also.