I've owned the Sigma 600mm f/8 mirror lens (got it for $50!), as well as Sigma's 500mm f/7.2 "normal" (non-mirror) telephoto (which I sold many years ago, sadly). Used them both on Nikons. The 600mm behaved just like any other mirror lens I've used, but it did seem sharper than some cheap 500mm mirror lenses. I did use it on a tripod exclusively. I liked the 500mm f/7.2 Sigma better, if only because I found the lack of an aperture control on the mirror lens limiting. I sold the mirror lens and am now looking for another long tele. Wish I hadn't sold the 500mm - it seems to be hard to find.