The trail from Caverne Dufour down to Cilaos is a real pain in the ass. I've seen many trails in iceland, norway, alaska and the european alps...this one is by far the worst! It is not the height difference itself but rather the way the trail is maintained. Thousands of wooden (and slippery) staircases especially at the end of the trail get your nerves. When you are finally in Cilaos you hear your knees say "Thank you!". I defenitely recommend hiking poles for the way down.
Cilaos itself is a very nice mountain village and is famous for its thermal springs. We didn't have the time to test the springs since we had our accomodation in Bras Sec, a neighboring community, but it must be a relaxed feeling after such a hike to bath in these springs. Cilaos also has a nice visitor center which was the only one we found on Reunion where english was spoken and understood. It is also the first adress to find out about current trail conditions and to make reservations for the mountain huts.