The Luminous Landscape has posted a review of an intriguing lens — Sony's 500mm f/8 mirror telephoto. It differs from other mirror telephotos in that it can autofocus. And mounted on a Sony DSLR, it's image-stabilized.
I borrowed a Sigma 600mm f/8 mirror lens from my Dad a year or two ago (he has yet to ask for it back). It'll get you in close, but manual focus is a crapshoot. It requires at the least a monopod, preferably a tripod to get anything good. Last week I hauled it out to try and take a picture of the moon, and ended up getting much better results with my 100-300mm Canon lens at 300mm, f/5.6, and a 2x teleconverter.
I find it interesting that Sony would even attempt to make this lens. I guess it allows them to say "750mm equivalent" for hundreds or thousands of dollars less than major brand lenses of the same length, and it adds a lens to Sony's small (but growing) lens lineup. Just a guess, though. The problem with that line of thinking is that it's the kind of message aimed at less experienced amateurs, who may or may not possess the patience it requires to work with this type of lens.
The Luminous Landscape has fairly nice things to say about it, so it must be an improvement over the one I've used. It's an interesting product for sure.