With the focal lenght of the mirror lenses (400, 500mm...) and the maximum aperture they get... They cant be very useable...
Their advantage, besides being cheaper of course, is they're relatively small (or at least, short) for the focal length, and relatively lightweight, so they're useful if you need to hike distances and take wildlife pictures. I used to use a Sigma 600mm mirror lens years back, with film.
But as you say, not very usable generally. Fixed aperture, usually something like f8, so narrow depth of field given the focal length.
The tendency to produce ugly doughnut rings was really only a problem if there were out of focus highlights in the background, say when taking pictures of seabirds with the sun glinting on water behind the subject or some such. Otherwise it could be quite effective.