I've got the 650-1300 and the 2x converter that goes with it. All in all, it's mostly a novelty. I picked it up used for somewhere under US$175 and for that price it can be fun to play with and the size certainly attracts attention. But if you want quality photos, this isn't going to give them to you. It needs an extremely stable tripod. Like, made of 12 tons of granite. It is highly susceptible to temperature based focus shifts. Everything makes the image jiggle. If you use it to shoot the moon, you will have to focus constantly because it will focus past the moon and apparently the DOF at moon distance is less than the diameter of the moon. But it is pretty amazing to be able to take recognizable pictures of things that you can barely see with the naked eye.
Here's the Golden Dome at Notre Dame University from 4 miles away (along with a bunch of sensor dirt) at 2600mm on a 60D:
Same shot, but with a 70mm:
And here's the moon at 2600mm:
I think if it were possible to stop it down a couple stops it might have acceptable sharpness. But you can't, and it doesn't.