A) there are a lot of people here who seem to be dogging the lens out who have obviously never tried it. As Keith stated, who seems to have some experience with these things, they're not totally awful.
B) They're basically good for shooting pictures of the moon or other stellar objects. (which isnt surprising given that they're essentially just telescopes that attach to a camera) It's going to take a LOT of trial and error to get your settings right, since it can't use your camera's metering system, this means you need a mostly static subject. You also generally want a black background, because of the weird background highlights effect Keith talked about.
A guy brought one of these things out to a local astrophotography meetup we had, and honestly the resulting shots he got of the moon were fairly okay. I mean it's not going to blow you away, and it is limited in what it can do, but for the price it's pretty good. And I think it got better results for astro photography than you'd get from a bridge camera, which tends to get really noisy in that type of atmosphere.
Now, would you use this as a general telephoto for 'normal' super telephoto type applications? No way. Trying to photograph wildlife with this thing would be a near disaster. You would NEVER get it in focus or properly exposed unless the animal was actually dead.