THIS guy is using one, the pictures looks "OK", the bokeh is REALLY strange, images are not so sharp but the lens reaches 800mm !
it costs $220 and I think it's actually worth it !!
I wonder if the lack of sharpness is due to technique? Some of the images look like they're exhibiting camera shake in some form. Even if there's an inherent lack of sharpness, that is pretty damn affordable for 800m.
And Mirror lenses are known for having "Doughnut Bokeh"
Edit: Not the 800mm but here's a frame with the 500mm f/6.3 on a 5DMKII
Most reviews I have read, say to pass on this (and other reflector-based lenses) as quality has been ditched in favour of magnification. Because of the construction of the lens and the reflector in it, the lens tends to only do well on bright, sunny days. And when you have ideal conditions, chances are that you may have to do some post-processing sharpening to get the pic where it should be.
Doughnut Bokeh
Also, the bokeh doughtnut will happen when you have bright points in your shot, but when your shot has other objects that aren't bright points, the result is OOF areas being doubled. Not pleasant at all. Most consider it distracting.
Example: Click Here
Incidently, if you're interested in taking pictures of the moon, It's really good for that!
Also... that lens' aperture is fixed. It's F/6.3 no matter what.
+1 for iso1600 recommending the Tamron adaptall mirror lenses. Jos-comment about the fixed aperture, is not correct-you can change it and the depth of field(focus) with a simple disc mounted to lens front(Waterhouse-stop). This will of course make the lens nearly impossible to focus, i guess a trade off .
