I was scared away when I read some rather bad reviews... it would be nice to hear some more thoughts though from people who have used them.

I bought a Spiratone 400mm off ebay a couple of years ago for about $80.00, it came with rear screw in filters, and a 2x doubler.  It supposedly was sold in the late 1960's and early 1970's.  I had to add a t mount for my canon.  It has some fungus growing on the rear element edges, but it never seemed to effect the pictures somehow

I came out with some OK pictures of landscapes and wading birds, but it was manual focus.  My dad had a 500mm sigma mirror for a while, and it seemed to be much better quality.  I never seemed to see the typical mirror lens "donuts" in the background blur that many people report as a problem.

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.