I have a 'Sigma 600mm f8 mirror', with a recently damaged mirror, so it's images are far from perfect, but can be useful despite this with a bit of EF juggling and glare avoidance, its a pleasure to carry and hand hold. I would love to find a mint condition one, as I would really like to replace mine. The 2m 1:3 macro is very handy too.
I really like using this lens, despite its horrible deformities.
However I also purchased a Japanese 'Pentax 1000mm f11 reflex' recently, has a little dust in it, but otherwise excellent condition. Very heavy, but built like a tank, and image quality is vastly superior to the poor old beat up sigma, and very acceptable for most purposes IMO.
Catadioptric lenses tend to polarize people very strongly; they either hate or love them. Personally I think they are an excellent alternative to spending 10 times as much on a non-reflex lens that weighs a lot more, and that you would NEVER be able to use in the situations I use a 'cat'.
I love them. I wish Sigma/Pentax would revive their cat manufacturing departments so I can buy one new, and maybe with all the new electronica like AF etc.
Theres a lot of good examples on flickr.. i think I am in most of the 'cat' groups:
http://flic.kr/betlog
But most of my images go here:
http://oh-hi.info
..and pretty much everything I have shot for weeks is hand-held with the 600mm or the 1000mm (yes really). I have a monopod as of tonight, so in theory my keep:delete ratio should improve. :)