Some are not very good, but others are. I currently have 4 different mirror lenses, two are not great but the other two are very good. One of which is an AF mirror lens!
One thing is that they are extremely sensitive to shocks causing mirror misalignment - so if your lens has ever been dropped, it’s probably lower performance than before due to the mirrors being out of wack.
One thing is that they are extremely sensitive to shocks causing mirror misalignment - so if your lens has ever been dropped, it’s probably lower performance than before due to the mirrors being out of wack.