Mirror Lenses?

Is anyone happy (or at least satisfied) with their cheap 500mm f8 manual focus mirror lens? My longest lens is just 300mm and I couldn't justify spending big bucks to go for a longer lens that I probably wouldn't use that often. If I had one, I would probably only use it for zoo shots. A couple of months ago, someone posted some nice looking shots taken with a Sigma 600mm mirror lens but even that poster seemed unsure whether he liked the lens.

I know I can use a TC with my 300mm but I could that with the 500mm or 600mm lens too.