I have no knowledge of the Opteka lens, from its general appearance it looks like the Centon 500 mm mirror lens that Jessops sell (or used to sell). I had one of these once, it was horrible, there was a correctly-exposed circle in the middle of the image falling off to about 2 stops underexposure at the edges of the picture.
I subsequently had a Nikon 500 mirror, which was very good - I sold it to my friend the local camera dealer and he also got great results on an African photo safari. Later I acquired a Tamron SP 500, this was also good and had the advantage of the Tamron Adaptall mount, which meant it could be used with Nikon, Pentax K and M42 (and anything else you care to name). I still have this and might even part with it for a certain figure!
It is always difficult, I think, to judge the definition of a 500 f8 mirror lens - the small aperture makes focusing tricky, you of course cannot stop down to increase depth of field, and any vibration (even with the camera on a tripod) will knock the edge off the sharpness. The big advantage, of course, is lack of crippling weight and price tag!