Trou de Fer is probably one of the most spectacular sights of La Reunion. You'll get a birds eye view of the end of the very deep and narrow gorge Bras de Caverne. As far as I know this is the only way to see Trou de Fer without having to rent a helicopter tour.
The trail to the viewpoint runs through dense mountain rain forest so it is extremely wet, muddy and slippery. At some points (especially shortly before the end) the trail has been made more accessible and hikable by the forest administration. The complete hike (roundtrip) will take you about 4-6 hours (starting at the parking lot just before the gate on the road to the Gite de Belouve), depending on your speed and the number of photostops you'll certainly make in the beautiful forest. If you start directly from the mountain hut Gite de Belouve it is certainly a bit shorter. Best time to do the hike is early in the morning when the gorge is free of clouds.