They’re OK for keeping very light road spray at bay, if the roads are wet, as mentioned, you’re better off with something bigger. I wouldn’t use one off road, it would be a waste of time.
They don’t fit easily on all saddles, so may need some modification with scissors. They don’t work well with seat packs. I bought one a couple of years ago, used it a couple of times and it’s now in the garage somewhere, I might even had binned it.