I have the Sigma 170-500mm APO lens, but not the DG version. It's a pretty OK lens for the money. It's not as sharp as I would like it to be, but with good technique, it is a very capable lens. The AF is VERY slow, and kind of loud on mine. It is a few years old tho, so maybe the newer ones are better. I have found that the AF hunts quite regularly, even in good light so manual focusing with this lens is pretty common.

The Opteka is so cheap because it is a mirror lens. I don't know much about them, but they have been discussed here a few times. Here are a couple of threads that talk about them.

Mirror Lenses?

Mirror Lense