I wear TLD Raid knee pads and generally have been very pleased with them. They’ve saved my knees more than once. The one drawback, I guess, is they slip on so I can’t put them on with shoes on, but that’s not a big deal to me since I put my shoes on at the TH anyway.
Edit: To address some of your other questions, I wear them on any trail ride, long or short and whether or not I’m descending. The only exceptions are a novice group ride I sometimes join for the company of the people in the group and in the winter if there’s soft new snow.