anyone ever use these?

[gokenin]

was browsing ebay this afternoon and came across this thing
http://cgi.ebay.com/Opteka-500mm-1000mm-Mirror-Lens-Pentax-K110D-K100D-K10D_W0QQitemZ280171662757QQihZ018QQcategoryZ48558 QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
what is a mirror lens and how does it work? seems like something very cheap for long distance so wondering if it horrible image quality:confused3 just wondering if anyone had any knowledge

[MICKEY88]

for starters your max aperture is f8 .... that alone would keep it out of my lens collection

[ukcatfan]

I have seen some decent shots with them, but they are not going to be the easiest to use. They will be MF only due to the small aperture. Also, points of light become donut shaped. Probably not bad for daylight, but I would not recommend for anything less than ideal lighting.

Kevin