They are superb. I was thinking, how can I take pictures of the night sky. Id love to zoom in loads onto planetary objects.
I only have a 75 - 300mm lens however.
To start you dont need more than that to do some astronomy. Just a tripod, remote release and be prepared for very long exposure times.
Being well away from built up areas helps too. If you really want to splash out you need an 8"+ telescope and adaptor to attach your camera.