I am probably highly biased towards this site. I grew up in an area rich in fossils and have spent many hours of my life in damp quarries sifting through rubble looking for them, as well as countless more hours in museums looking at them. The Messel Pit is undoubtedly one of the most famous, important and productive fossil sites on Earth - it is crucial to our understanding of life in the early Eocene (~50 million years ago), particularly in regards to the evolution and rise of mammals. The Messel Pit was one of the WHS in Germany that I was most excited about visiting.

That said, if you are not a fossil enthusiast then fossil sites tend to be unimpressive to look at. The Messel Pit is no exception. Without the visitor centre it would literally be just a hole in the ground. The visitor centre is small and the €7 fee felt steep for what you get, although it is a nice introduction to the site and why it is so important. Regardless, I am personally more than happy to put some money towards protecting and developing a site as important as this. My German isn't great so I didn't do a tour, although I note that they do have a single public tour in English at 1530 on the first Friday of every month. The site was pretty easy to get to on the bus from Darmstadt, and they even run on Sundays.

While not part of the WHS, I highly recommend the Museumshof in Messel itself. This is a small community run museum that is free to visit and staffed by passionate volunteers. They have a variety of wonderful fossils on display, as well as some local history. I was the first visitor of the day and the friendly volunteer took me around personally, showed me his favourite fossils, and translated some of the displays for me. Even with my enthusiasm for fossils I only spent about 45 minutes here, but this museum was the highlight of my whole weekend in Darmstadt/Messel. I would love it to get more visitors and support. The Hessisches Landesmuseum in Darmstadt also has a great permanent exhibition where many of the best Messel fossils are on display, and is close to the Mathildenhöhe WHS.