The cheaper mirror lenses are not generally very good as far as I have heard. The Tamron adaptall-2 500 mm f/8 lens is supposed to be good. I have their 350mm f/5.6 mirror lens and it is very fine. But that means an adapatll-2 pentax K mount adapter and the 500mm lens (likely $200. from KEH). There are two versions of the 500 mm lens--older one has a tripod mount on the lens--newer one not. If you plan to use a TC and/or extension tubes, then I suggest you need the older one with a tripod mount on the lens. (On the other hand I have heard the newer one is better--read the reviews here--also there is an adaptall-2 discussion group here on the pentaxforums--I believe there may be a mirror lens group as well.)
Yes mirror lens can be excellent for macro, likely to m=0.5 anyway [ which likely means addition of about 125 mm extension, or a 2x Teleconverter (TC)]. I actually bought it partly/largely for this use--the odd doughnut shaped out of focus highlights are generally not as big a problem w/ macro--and the long focal length means a large focusing distance--so skittish insects, etc. may not be so skittish.
But using a mirror lens is very hard to do--it means your camera technique must be good, high iso, usually tripod w/ SR off, not shooting over hot surfaces which cause atmospheric turbulence, etc. Many people give up and say they are NG--but it is often they don't take the care needed.
Last edited by dms; 02-02-2014 at 08:58 PM.